AD636KCHIP [ADI]

IC RMS TO DC CONVERTER, 0.1 MHz, UUC11, Analog Special Function Converter;


型号：	AD636KCHIP
厂家：	ADI
描述：	IC RMS TO DC CONVERTER, 0.1 MHz, UUC11, Analog Special Function Converter 转换器
文件：	总8页 (文件大小：161K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

Low Level,

True RMS-to-DC Converter

AD636

PIN CONNECTIONS &

FEATURES

True RMS-to-DC Conversion

200 mV Full Scale

FUNCTIONAL BLOCK DIAGRAM

Laser-Trimmed to High Accuracy

0.5% Max Error (AD636K)

1.0% Max Error (AD636J)

OUT

BUF IN

ABSOLUTE

VALUE

Wide Response Capability:

Computes RMS of AC and DC Signals

1 MHz –3 dB Bandwidth: V RMS >100 mV

Signal Crest Factor of 6 for 0.5% Error

dB Output with 50 dB Range

Low Power: 800 ␮A Quiescent Current

Single or Dual Supply Operation

Monolithic Integrated Circuit

Low Cost

10k⍀

–

AD636

BUF

AD636

BUF OUT

COMMON

12 NC

SQUARER

DIVIDER

–V

CURRENT

MIRROR

10k⍀

SQUARER

DIVIDER

CURRENT

MIRROR

BUF OUT

BUF IN

COMMON

ABSOLUTE

VALUE

10k⍀

BUF

–

OUT

10k⍀

–V

NC = NO CONNECT

Available in Chip Form

PRODUCT DESCRIPTION

The AD636 is a low power monolithic IC which performs true

rms-to-dc conversion on low level signals. It offers performance

which is comparable or superior to that of hybrid and modular

converters costing much more. The AD636 is specified for a

signal range of 0 mV to 200 mV rms. Crest factors up to 6 can

be accommodated with less than 0.5% additional error, allowing

accurate measurement of complex input waveforms.

is accurate within ±0.2 mV to ±0.3% of reading. Both versions

are specified for the 0°C to +70°C temperature range, and are

offered in either a hermetically sealed 14-pin DIP or a 10-lead

TO-100 metal can. Chips are also available.

PRODUCT HIGHLIGHTS

1. The AD636 computes the true root-mean-square of a com-

plex ac (or ac plus dc) input signal and gives an equivalent dc

output level. The true rms value of a waveform is a more

useful quantity than the average rectified value since it is a

measure of the power in the signal. The rms value of an

ac-coupled signal is also its standard deviation.

The low power supply current requirement of the AD636, typi-

cally 800 µA, allows it to be used in battery-powered portable

instruments. A wide range of power supplies can be used, from

±2.5 V to ±16.5 V or a single +5 V to +24 V supply. The input

and output terminals are fully protected; the input signal can

exceed the power supply with no damage to the device (allowing

the presence of input signals in the absence of supply voltage)

and the output buffer amplifier is short-circuit protected.

2. The 200 millivolt full-scale range of the AD636 is compatible

with many popular display-oriented analog-to-digital con-

verters. The low power supply current requirement permits

use in battery powered hand-held instruments.

The AD636 includes an auxiliary dB output. This signal is

derived from an internal circuit point which represents the loga-

rithm of the rms output. The 0 dB reference level is set by an

externally supplied current and can be selected by the user

to correspond to any input level from 0 dBm (774.6 mV) to

–20 dBm (77.46 mV). Frequency response ranges from 1.2 MHz

at a 0 dBm level to over 10 kHz at –50 dBm.

3. The only external component required to perform measure-

ments to the fully specified accuracy is the averaging capaci-

tor. The value of this capacitor can be selected for the desired

trade-off of low frequency accuracy, ripple, and settling time.

4. The on-chip buffer amplifier can be used to buffer either the

input or the output. Used as an input buffer, it provides

accurate performance from standard 10 MΩ input attenua-

tors. As an output buffer, it can supply up to 5 milliamps of

output current.

The AD636 is designed for ease of use. The device is factory-

trimmed at the wafer level for input and output offset, positive

and negative waveform symmetry (dc reversal error), and full-

scale accuracy at 200 mV rms. Thus no external trims are re-

quired to achieve full-rated accuracy.

5. The AD636 will operate over a wide range of power supply

voltages, including single +5 V to +24 V or split ±2.5 V to

±16.5 V sources. A standard 9 V battery will provide several

hundred hours of continuous operation.

AD636 is available in two accuracy grades; the AD636J total

error of ±0.5 mV ±0.06% of reading, and the AD636K

REV. B

Information furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assumed by Analog Devices for its

use, nor for any infringements of patents or other rights of third parties

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781/329-4700

Fax: 781/326-8703

World Wide Web Site: http://www.analog.com

(@ +25؇C, and +V_S= +3 V, –V_S= –5 V, unless otherwise noted)

AD636–SPECIFICATIONS

Model

AD636J

Typ

AD636K

Typ

Min

Max

Min

Max

Units

TRANSFER FUNCTION

V_OUT

avg. (V_IN

)

V_OUT

avg. (V_IN)

CONVERSION ACCURACY

Total Error, Internal Trim^{1, 2}

vs. Temperature, 0°C to +70°C

vs. Supply Voltage

؎0.5 ؎1.0

±0.1 ±0.01

؎0.2 ؎0.5

±0.1 ±0.005

mV ±% of Reading

mV ±% of Reading/°C

mV ±% of Reading/V

% of Reading

±0.1 ±0.01

±0.2

±0.3 ±0.3

±0.1 ±0.01

±0.1

±0.1 ±0.2

dc Reversal Error at 200 mV

Total Error, External Trim¹

mV ±% of Reading

ERROR VS. CREST FACTOR³

Crest Factor 1 to 2

Specified Accuracy

Crest Factor = 3

–0.2

–0.5

–0.2

–0.5

% of Reading

Crest Factor = 6

AVERAGING TIME CONSTANT

ms/µF CAV

INPUT CHARACTERISTICS

Signal Range, All Supplies

Continuous rms Level

Peak Transient Inputs

+3 V, –5 V Supply

0 to 200

mV rms

±2.8

±2.0

±5.0

±2.8

±2.0

±5.0

V pk

±2.5 V Supply

±5 V Supply

Maximum Continuous Nondestructive

Input Level (All Supply Voltages)

Input Resistance

±12

±0.5

±12

±0.2

V pk

kΩ

5.33

6.67

5.33

6.67

Input Offset Voltage

FREQUENCY RESPONSE^{2, 4}

Bandwidth for 1% Additional Error (0.09 dB)

V_IN= 10 mV

V_IN= 100 mV

V_IN= 200 mV

kHz

130

±3 dB Bandwidth

_IN= 10 mV

100

900

1.5

100

900

1.5

kHz

MHz

_IN= 100 mV

V_IN= 200 mV

OUTPUT CHARACTERISTICS²

Offset Voltage, V_IN= COM

vs. Temperature

؎0.5

؎0.2

µV/°C

mV/ V

±10

vs. Supply

±0.1

Voltage Swing

+3 V, –5 V Supply

±5 V to ±16.5 V Supply

Output Impedance

0.3

0 to +1.0

0.3

0 to +1.0

kΩ

dB OUTPUT

Error, V_IN= 7 mV to 300 mV rms

Scale Factor

Scale Factor Temperature Coefficient

±0.3

–3.0

+0.33

–0.033

؎0.5

±0.1

–3.0

+0.33

–0.033

؎0.2

mV/dB

% of Reading/°C

dB/°C

µA

_REFfor 0 dB = 0.1 V rms

I_REFRange

I_OUTTERMINAL

_OUTScale Factor

100

µA/V rms

_OUTScale Factor Tolerance

–20

±10

+20

–20

±10

+20

Output Resistance

kΩ

Voltage Compliance

–V_Sto (+V_S

–2 V)

–V_Sto (+V_S

–2 V)

BUFFER AMPLIFIER

Input and Output Voltage Range

–V_Sto (+V_S

–2 V)

–V_Sto (+V_S

–2 V)

Input Offset Voltage, R_S= 10k

Input Bias Current

Input Resistance

±0.8

100

10⁸

؎2

300

±0.5

100

10⁸

؎1

300

Ω

Output Current

(+5 mA,

–130 µA)

(+5 mA,

–130 µA)

Short Circuit Current

Small Signal Bandwidth

Slew Rate⁵

MHz

V/µs

POWER SUPPLY

Voltage, Rated Performance

Dual Supply

+3, –5

0.80

+3, –5

0.80

+2, –2.5

±16.5

+24

1.00

+2, –2.5

±16.5

+24

1.00

Single Supply

Quiescent Current⁶

–2–

REV. B

AD636

Model

AD636J

Typ

AD636K

Typ

Min

Max

Min

Max

Units

TEMPERATURE RANGE

Rated Performance

Storage

–55

+70

+150

–55

+70

+150

°C

TRANSISTOR COUNT

NOTES

¹Accuracy specified for 0 mV to 200 mV rms, dc or 1 kHz sine wave input. Accuracy is degraded at higher rms signal levels.

²Measured at Pin 8 of DIP (I_OUT), with Pin 9 tied to common.

³Error vs. crest factor is specified as additional error for a 200 mV rms rectangular pulse trim, pulse width = 200 µs.

⁴Input voltages are expressed in volts rms.

⁵With 10 kΩ pull down resistor from Pin 6 (BUF OUT) to –V_S.

⁶With BUF input tied to Common.

Specifications subject to change without notice.

All min and max specifications are guaranteed. Specifications shown in boldface are tested on all production units at final electrical test and are used to calculate outgoing

quality levels.

ABSOLUTE MAXIMUM RATINGS¹

ORDERING GUIDE

Supply Voltage

Temperature Package

Package

Options

Dual Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±16.5 V

Single Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +24 V

Internal Power Dissipation². . . . . . . . . . . . . . . . . . . .500 mW

Maximum Input Voltage . . . . . . . . . . . . . . . . . . . . ±12 V Peak

Storage Temperature Range N, R . . . . . . . . . –55°C to +150°C

Operating Temperature Range

AD636J/K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C

Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300°C

ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000 V

Model

Range

Descriptions

AD636JD

AD636KD

AD636JH

AD636KH

AD636J Chip

AD636JD/+

0°C to +70°C

Side Brazed Ceramic DIP D-14

Header

Chip

H-10A

Side Brazed Ceramic DIP D-14

NOTES

STANDARD CONNECTION

¹Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent damage to the device. This is a stress rating only; functional operation of the

device at these or any other conditions above those indicated in the operational

section of this specification is not implied. Exposure to absolute maximum rating

conditions for extended periods may affect device reliability.

²10-Lead Header: θ_JA= 150°C/Watt.

The AD636 is simple to connect for the majority of high accu-

racy rms measurements, requiring only an external capacitor to

set the averaging time constant. The standard connection is

shown in Figure 1. In this configuration, the AD636 will mea-

sure the rms of the ac and dc level present at the input, but will

show an error for low frequency inputs as a function of the filter

capacitor, C_AV, as shown in Figure 5. Thus, if a 4 µF capacitor

is used, the additional average error at 10 Hz will be 0.1%, at

3 Hz it will be 1%. The accuracy at higher frequencies will be

according to specification. If it is desired to reject the dc input, a

capacitor is added in series with the input, as shown in Fig-

ure 3; the capacitor must be nonpolar. If the AD636 is driven

with power supplies with a considerable amount of high frequency

ripple, it is advisable to bypass both supplies to ground with

0.1 µF ceramic discs as near the device as possible. C_Fis an

optional output ripple filter, as discussed elsewhere in this data

sheet.

14-Lead Side Brazed Ceramic DIP: θ_JA= 95°C/Watt.

METALIZATION PHOTOGRAPH

Contact factory for latest dimensions.

Dimensions shown in inches and (mm).

0.1315 (3.340)

COM

+V 14

8 I

OUT

–

0.0807

(2.050)

(OPTIONAL)

10k⍀

ABSOLUTE

VALUE

–

1a*

1b*

7 BUF IN

BUF

AD636

OUT

6 BUF OUT

CURRENT

MIRROR

AD636

–V

SQUARER

DIVIDER

10k⍀

–V

SQUARER

DIVIDER

PAD NUMBERS CORRESPOND TO PIN NUMBERS

FOR THE TO-116 14-PIN CERAMIC DIP PACKAGE.

CURRENT

MIRROR

ABSOLUTE

VALUE

OUT

NOTE

*BOTH PADS SHOWN MUST BE CONNECTED TO V

10k⍀

BUF

–

10k⍀

(OPTIONAL)

–V

–

Figure 1. Standard RMS Connection

REV. B

–3–

AD636

APPLYING THE AD636

flows into Pin 10 (Pin 2 on the “H” package). Alternately, the

COM pin of some CMOS ADCs provides a suitable artificial

ground for the AD636. AC input coupling requires only capaci-

tor C2 as shown; a dc return is not necessary as it is provided

internally. C2 is selected for the proper low frequency break

point with the input resistance of 6.7 kΩ; for a cut-off at 10 Hz,

C2 should be 3.3 µF. The signal ranges in this connection are

slightly more restricted than in the dual supply connection. The

load resistor, R_L, is necessary to provide current sinking capability.

The input and output signal ranges are a function of the supply

voltages as detailed in the specifications. The AD636 can also

be used in an unbuffered voltage output mode by disconnecting

the input to the buffer. The output then appears unbuffered

across the 10 kΩ resistor. The buffer amplifier can then be used

for other purposes. Further, the AD636 can be used in a current

output mode by disconnecting the 10 kΩ resistor from the

ground. The output current is available at Pin 8 (Pin 10 on the

“H” package) with a nominal scale of 100 µA per volt rms input,

positive out.

–

3.3␮F

OPTIONAL TRIMS FOR HIGH ACCURACY

If it is desired to improve the accuracy of the AD636, the exter-

nal trims shown in Figure 2 can be added. R4 is used to trim the

offset. The scale factor is trimmed by using R1 as shown. The

insertion of R2 allows R1 to either increase or decrease the scale

factor by ±1.5%.

ABSOLUTE

VALUE

NONPOLARIZED

0.1␮F

AD636

SQUARER

DIVIDER

20k⍀

The trimming procedure is as follows:

CURRENT

MIRROR

1. Ground the input signal, V_IN, and adjust R4 to give zero

volts output from Pin 6. Alternatively, R4 can be adjusted to

give the correct output with the lowest expected value of V_IN.

2. Connect the desired full-scale input level to V_IN, either dc or

a calibrated ac signal (1 kHz is the optimum frequency);

then trim R1 to give the correct output from Pin 6, i.e.,

200 mV dc input should give 200 mV dc output. Of course,

a ±200 mV peak-to-peak sine wave should give a 141.4 mV

dc output. The remaining errors, as given in the specifica-

tions, are due to the nonlinearity.

0.1␮F

OUT

10k⍀

BUF

–

39k⍀

10k⍀ to 1k⍀

10k⍀

Figure 3. Single Supply Connection

CHOOSING THE AVERAGING TIME CONSTANT

The AD636 will compute the rms of both ac and dc signals. If

the input is a slowly-varying dc voltage, the output of the AD636

will track the input exactly. At higher frequencies, the average

output of the AD636 will approach the rms value of the input

signal. The actual output of the AD636 will differ from the ideal

output by a dc (or average) error and some amount of ripple, as

demonstrated in Figure 4.

–

SCALE

FACTOR

ADJUST

ABSOLUTE

VALUE

200⍀

؎1.5%

AD636

–V

SQUARER

DIVIDER

IDEAL

DC ERROR = E – E (IDEAL)

CURRENT

MIRROR

154⍀

–V

AVERAGE E = E

OUT

DOUBLE-FREQUENCY

RIPPLE

500k⍀

10k⍀

BUF

470k⍀

–

TIME

10k⍀

OFFSET

ADJUST

Figure 4. Typical Output Waveform for Sinusoidal Input

The dc error is dependent on the input signal frequency and the

value of C_AV. Figure 5 can be used to determine the minimum

value of C_AVwhich will yield a given % dc error above a given

frequency using the standard rms connection.

Figure 2. Optional External Gain and Output Offset Trims

SINGLE SUPPLY CONNECTION

The applications in Figures 1 and 2 assume the use of dual

power supplies. The AD636 can also be used with only a single

positive supply down to +5 volts, as shown in Figure 3. Figure 3

is optimized for use with a 9 volt battery. The major limitation

of this connection is that only ac signals can be measured since

the input stage must be biased off ground for proper operation.

This biasing is done at Pin 10; thus it is critical that no extrane-

ous signals be coupled into this point. Biasing can be accom-

plished by using a resistive divider between +V_Sand ground.

The values of the resistors can be increased in the interest of

lowered power consumption, since only 1 microamp of current

The ac component of the output signal is the ripple. There are

two ways to reduce the ripple. The first method involves using

a large value of C_AV. Since the ripple is inversely proportional

to C_AV, a tenfold increase in this capacitance will effect a tenfold

reduction in ripple. When measuring waveforms with high crest

factors, (such as low duty cycle pulse trains), the averaging time

constant should be at least ten times the signal period. For

example, a 100 Hz pulse rate requires a 100 ms time constant,

which corresponds to a 4 µF capacitor (time constant = 25 ms

per µF).

–4–

REV. B

AD636

100

ABSOLUTE

VALUE

AD636

SQUARER

DIVIDER

–V

–

CURRENT

MIRROR

1.0

0.1

0.01

10k⍀

VALUES FOR C AND

BUF

–

(FOR SINGLE POLE, SHORT Rx,

REMOVE C3)

1% SETTLING TIME FOR

STATED % OF READING

AVERAGING ERROR*

ACCURACY ؎20% DUE TO

COMPONENT TOLERANCE

10k⍀

0.1

–

10k⍀

*% dc ERROR + % RIPPLE (PEAK)

0.01

OUT

rms

100

10k

100k

INPUT FREQUENCY – Hz

Figure 7. 2 Pole ‘’Post’’ Filter

Figure 5. Error/Settling Time Graph for Use with the

Standard rms Connection

The primary disadvantage in using a large C_AVto remove ripple

is that the settling time for a step change in input level is in-

creased proportionately. Figure 5 shows the relationship be-

tween C_AVand 1% settling time is 115 milliseconds for each

microfarad of C_AV. The settling time is twice as great for de-

creasing signals as for increasing signals (the values in Figure 5

are for decreasing signals). Settling time also increases for low

signal levels, as shown in Figure 6.

p-p RIPPLE

(ONE POLE)

= 1␮F

p-p RIPPLE

= 1␮F (FIG 1)

C2 = 4.7␮F

DC ERROR

= 1␮F

(ALL FILTERS)

p-p RIPPLE

(TWO POLE)

= 1␮F, C2 = C3 = 4.7␮F

10.0

0.1

100

FREQUENCY – Hz

10k

7.5

5.0

2.5

Figure 8. Performance Features of Various Filter Types

RMS MEASUREMENTS

AD636 PRINCIPLE OF OPERATION

The AD636 embodies an implicit solution of the rms equation

that overcomes the dynamic range as well as other limitations

inherent in a straightforward computation of rms. The actual

computation performed by the AD636 follows the equation:

1.0

1mV

10mV

100mV

rms INPUT LEVEL

V_IN

Figure 6. Settling Time vs. Input Level

V rms = Avg.

V rms

A better method for reducing output ripple is the use of a

“post-filter.” Figure 7 shows a suggested circuit. If a single pole

filter is used (C3 removed, R_Xshorted), and C2 is approxi-

mately 5 times the value of C_AV, the ripple is reduced as shown

in Figure 8, and settling time is increased. For example, with

C_AV= 1 µF and C2 = 4.7 µF, the ripple for a 60 Hz input is re-

duced from 10% of reading to approximately 0.3% of reading.

The settling time, however, is increased by approximately a

factor of 3. The values of C_AVand C2 can therefore be reduced

to permit faster settling times while still providing substantial

ripple reduction.

The two-pole post-filter uses an active filter stage to provide

even greater ripple reduction without substantially increasing

the settling times over a circuit with a one-pole filter. The values

of C_AV, C2, and C3 can then be reduced to allow extremely fast

settling times for a constant amount of ripple. Caution should

be exercised in choosing the value of C_AV, since the dc error is

dependent upon this value and is independent of the post filter.

For a more detailed explanation of these topics refer to the

RMS-to-DC Conversion Application Guide, 2nd Edition, available

from Analog Devices.

Figure 9 is a simplified schematic of the AD636; it is subdivided

into four major sections: absolute value circuit (active rectifier),

squarer/divider, current mirror, and buffer amplifier. The input

voltage, V_IN, which can be ac or dc, is converted to a unipolar

current I₁, by the active rectifier A₁, A₂. I₁drives one input of

the squarer/divider, which has the transfer function:

I₁²

I₃

I₄=

The output current, I₄, of the squarer/divider drives the current

mirror through a low-pass filter formed by R1 and the externally

connected capacitor, C_AV. If the R1, C_AVtime constant is much

greater than the longest period of the input signal, then I₄is

effectively averaged. The current mirror returns a current I₃,

which equals Avg. [I₄], back to the squarer/divider to complete

the implicit rms computation. Thus:

I₁

I₄= Avg.

= I₁rms

I₄

REV. B

–5–

AD636

The current mirror also produces the output current, I_OUT

Addition of an external resistor in parallel with R_Ealters this

voltage divider such that increased negative swing is possible.

which equals 2I₄. I_OUTcan be used directly or converted to a

voltage with R2 and buffered by A₄to provide a low impedance

voltage output. The transfer function of the AD636 thus results:

Figure 11 shows the value of R_EXTERNALfor a particular ratio of

V_PEAKto –V_Sfor several values of R_LOAD.Addition, of R_EXTERNAL

increases the quiescent current of the buffer amplifier by an

amount equal to R_EXT/–V_S. Nominal buffer quiescent current

with no R_EXTERNALis 30 µA at –V_S= –5 V.

V_OUT= 2 R2 I rms = V_INrms

The dB output is derived from the emitter of Q₃, since the volt-

age at this point is proportional to –log V_IN. Emitter follower,

Q₅, buffers and level shifts this voltage, so that the dB output

voltage is zero when the externally supplied emitter current

(I_REF) to Q₅approximates I₃.

1.0

CURRENT MIRROR

= 50k⍀

COM

0.5

20␮A

25k⍀

10␮A

= 16.7k⍀

ABSOLUTE VALUE/

VOLTAGE–CURRENT

CONVERTER

AV OUT

10k⍀

REF

OUT

BUF

= 6.7k⍀

– ⍀

20k⍀

IN BUFFER

BUF

OUT

10k

100k

8k⍀

Q2 Q4

EXTERNAL

10k⍀

8k⍀

Figure 11. Ratio of Peak Negative Swing to –V_Svs.

_EXTERNALfor Several/Load Resistances

ONE-QUADRANT

SQUARER/

DIVIDER

–V

FREQUENCY RESPONSE

Figure 9. Simplified Schematic

THE AD636 BUFFER AMPLIFIER

The buffer amplifier included in the AD636 offers the user

additional application flexibility. It is important to understand

some of the characteristics of this amplifier to obtain optimum

The AD636 utilizes a logarithmic circuit in performing the

implicit rms computation. As with any log circuit, bandwidth is

proportional to signal level. The solid lines in the graph below

represent the frequency response of the AD636 at input levels

from 1 millivolt to 1 volt rms. The dashed lines indicate the

upper frequency limits for 1%, 10%, and ±3 dB of reading

additional error. For example, note that a 1 volt rms signal will

produce less than 1% of reading additional error up to 220 kHz.

A 10 millivolt signal can be measured with 1% of reading addi-

tional error (100 µV) up to 14 kHz.

performance. Figure 10 shows a simplified schematic of the buffer.

Since the output of an rms-to-dc converter is always positive, it

is not necessary to use a traditional complementary Class AB

output stage. In the AD636 buffer, a Class A emitter follower is

used instead. In addition to excellent positive output voltage

swing, this configuration allows the output to swing fully down

to ground in single-supply applications without the problems

associated with most IC operational amplifiers.

1 VOLT rms INPUT

10%

؎3dB

200mV rms INPUT

100mV rms INPUT

200m

100m

30mV rms INPUT

30m

10m

CURRENT

MIRROR

10mV rms

INPUT

BUFFER

OUTPUT

5␮A 5␮A

10k⍀

BUFFER

INPUT

LOAD

1mV rms INPUT

40k⍀

100␮

EXTERNAL

10k

100k

FREQUENCY – Hz

10M

–V

(OPTIONAL, SEE TEXT)

Figure 10. AD636 Buffer Amplifier Simplified Schematic

Figure 12. AD636 Frequency Response

When this amplifier is used in dual-supply applications as an

input buffer amplifier driving a load resistance referred to

ground, steps must be taken to insure an adequate negative

voltage swing. For negative outputs, current will flow from the

load resistor through the 40 kΩ emitter resistor, setting up a

voltage divider between –V_Sand ground. This reduced effective

–V_S, will limit the available negative output swing of the buffer.

AC MEASUREMENT ACCURACY AND CREST FACTOR

Crest factor is often overlooked in determining the accuracy of

an ac measurement. Crest factor is defined as the ratio of the

peak signal amplitude to the rms value of the signal (C.F. = V_P/

V rms) Most common waveforms, such as sine and triangle

waves, have relatively low crest factors (<2). Waveforms that

–6–

REV. B

AD636

resemble low duty cycle pulse trains, such as those occurring in

switching power supplies and SCR circuits, have high crest

factors. For example, a rectangular pulse train with a 1% duty

Circuit Description

The input voltage, V_IN, is ac coupled by C4 while resistor R8,

together with diodes D1, and D2, provide high input voltage

protection.

cycle has a crest factor of 10 (C.F. =

1 η

Figure 13 is a curve of reading error for the AD636 for a 200 mV

rms input signal with crest factors from 1 to 7. A rectangular

pulse train (pulse width 200 µs) was used for this test since it is

the worst-case waveform for rms measurement (all the energy is

contained in the peaks). The duty cycle and peak amplitude

were varied to produce crest factors from 1 to 7 while maintain-

ing a constant 200 mV rms input amplitude.

The buffer’s output, Pin 6, is ac coupled to the rms converter’s

input (Pin 1) by capacitor C2. Resistor, R9, is connected between

the buffer’s output, a Class A output stage, and the negative output

swing. Resistor R1, is the amplifier’s “bootstrapping” resistor.

With this circuit, single supply operation is made possible by

setting “ground” at a point between the positive and negative

sides of the battery. This is accomplished by sending 250 µA

from the positive battery terminal through resistor R2, then

through the 1.2 volt AD589 bandgap reference, and finally back

to the negative side of the battery via resistor R10. This sets

ground at 1.2 volts +3.18 volts (250 µA × 12.7 kΩ) = 4.4 volts

below the positive battery terminal and 5.0 volts (250 µA × 20 kΩ)

above the negative battery terminal. Bypass capacitors C3 and

C5 keep both sides of the battery at a low ac impedance to

ground. The AD589 bandgap reference establishes the 1.2 volt

regulated reference voltage which together with resistor R3 and

0.5

200␮s

␩ = DUTY CYCLE =

CF = 1/

␩

(rms) = 200mV

–0.5

–1.0

200␮s

trimming potentiometer R4 set the zero dB reference current I_REF

Performance Data

0 dB Reference Range = 0 dBm (770 mV) to –20 dBm

(77 mV) rms

0 dBm = 1 milliwatt in 600 Ω

Input Range (at I_REF= 770 mV) = 50 dBm

CREST FACTOR

Input Impedance = approximately 10¹⁰

Ω

Figure 13. Error vs. Crest Factor

V_SUPPLYOperating Range +5 V dc to +20 V dc

I_QUIESCENT= 1. 8 mA typical

A COMPLETE AC DIGITAL VOLTMETER

Figure 14 shows a design for a complete low power ac digital

voltmeter circuit based on the AD636. The 10 MΩ input

attenuator allows full-scale ranges of 200 mV, 2 V, 20 V and

200 V rms. Signals are capacitively coupled to the AD636 buffer

amplifier, which is connected in an ac bootstrapped configura-

tion to minimize loading. The buffer then drives the 6.7 kΩ

input impedance of the AD636. The COM terminal of the ADC

chip provides the false ground required by the AD636 for single

supply operation. An AD589 1.2 volt reference diode is used to

provide a stable 100 millivolt reference for the ADC in the lin-

ear rms mode; in the dB mode, a 1N4148 diode is inserted in

series to provide correction for the temperature coefficient of the

dB scale factor. Calibration of the meter is done by first adjust-

ing offset pot R17 for a proper zero reading, then adjusting the

R13 for an accurate readout at full scale.

Accuracy with 1 kHz sine wave and 9 volt dc supply:

0 dB to –40 dBm ± 0.1 dBm

0 dBm to –50 dBm ± 0.15 dBm

+10 dBm to –50 dBm ± 0.5 dBm

Frequency Response ؎3 dBm

Input

0 dBm = 5 Hz to 380 kHz

–10 dBm = 5 Hz to 370 kHz

–20 dBm = 5 Hz to 240 kHz

–30 dBm = 5 Hz to 100 kHz

–40 dBm = 5 Hz to 45 kHz

–50 dBm = 5 Hz to 17 kHz

Calibration

1. First calibrate the zero dB reference level by applying a 1 kHz

sine wave from an audio oscillator at the desired zero dB

amplitude. This may be anywhere from zero dBm (770 mV

rms – 2.2 volts p-p) to –20 dBm (77 mV rms 220 mV – p-p).

Adjust the I_REFcal trimmer for a zero indication on the analog

meter.

Calibration of the dB range is accomplished by adjusting R9 for

the desired 0 dB reference point, then adjusting R14 for the

desired dB scale factor (a scale of 10 counts per dB is convenient).

Total power supply current for this circuit is typically 2.8 mA

using a 7106-type ADC.

2. The final step is to calibrate the meter scale factor or gain.

A LOW POWER, HIGH INPUT IMPEDANCE dB METER

Introduction

Apply an input signal –40 dB below the set zero dB reference

and adjust the scale factor calibration trimmer for a 40 µA

reading on the analog meter.

The portable dB meter circuit featured here combines the func-

tions of the AD636 rms converter, the AD589 voltage reference,

and a µA776 low power operational amplifier. This meter offers

excellent bandwidth and superior high and low level accuracy

while consuming minimal power from a standard 9 volt transis-

tor radio battery.

The temperature compensation resistors for this circuit may be

purchased from: Tel Labs Inc., 154 Harvey Road, P.O. Box 375,

Londonderry, NH 03053, Part #Q332A 2 kΩ 1% +3500 ppm/°C

or from Precision Resistor Company, 109 U.S. Highway 22, Hill-

side, NJ 07205, Part #PT146 2 kΩ 1% +3500 ppm/°C.

In this circuit, the built-in buffer amplifier of the AD636 is used

as a “bootstrapped” input stage increasing the normal 6.7 kΩ

input Z to an input impedance of approximately 10¹⁰Ω.

REV. B

–7–

AD636

1N4148

–

47k⍀

2.2␮F

200mV

10%

1M⍀

OFF

ABSOLUTE

VALUE

–

0.02␮F

2.49k⍀

1N4148

1␮F

–V

3-1/2 DIGIT

7106 TYPE

A/D

9M⍀

AD636

R11

10k⍀

100k⍀

0dB SET

CONVERTER

SQUARER

DIVIDER

LIN

900k⍀

REF HI

R10

20k⍀

R12

1k⍀

–

BATTERY

20V

R14

6.8␮F

1.2V

AD589

REF LO

10k⍀

CURRENT

MIRROR

R13

500⍀

90k⍀

SCALE

COM

200V

LIN

10k⍀

LIN

SCALE

BUF

–

3-1/2

DIGIT

10k⍀

20k⍀

LCD

R15

1M⍀

DISPLAY

ANALOG

COM

0.01␮F

LIN

6.8␮F

1N4148

–V

LXD 7543

–V

Figure 14. A Portable, High Z Input, RMS DPM and dB Meter Circuit

3.3␮F

1N6263

1M⍀

ON/OFF

+4.4 VOLTS

+1.2 VOLTS

–

ABSOLUTE

VALUE

9 VOLT

12.7k⍀

6.8␮F

SCALE FACTOR

ADJUST

AD636

500k⍀

5k⍀

10␮F

SQUARER

DIVIDER

REF

SIGNAL

INPUT

10k⍀

AD589J

ADJUST

*R7

2k⍀

250␮A

100␮A

0.1␮F

–

CURRENT

MIRROR

100⍀

–

0–50␮A

47k⍀

1 WATT

␮A776

0.1␮F

10k⍀

10␮F

BUF

–

R10

20k⍀

10k⍀

R11

10k⍀

820k⍀

1N6263

+4.7 VOLTS

ALL RESISTORS 1/4 WATT 1% METAL FILM UNLESS OTHERWISE STATED EXCEPT

*WHICH IS 2k⍀ +3500ppm 1% TC RESISTOR.

Figure 15. A Low Power, High Input Impedance dB Meter

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

D Package (TO-116)

H Package (TO-100)

REFERENCE PLANE

0.750 (19.05)

0.500 (12.70)

0.185 (4.70)

0.165 (4.19)

0.005 (0.13) MIN

0.098 (2.49) MAX

0.160 (4.06)

0.110 (2.79)

0.250 (6.35) MIN

0.050 (1.27) MAX

0.310 (7.87)

0.220 (5.59)

0.320 (8.13)

0.290 (7.37)

0.045 (1.14)

0.027 (0.69)

0.115

PIN 1

0.785 (19.94) MAX

(2.92)

0.060 (1.52)

BSC

0.015 (0.38)

0.200 (5.08)

MAX

0.150

(3.81)

MAX

0.034 (0.86)

0.027 (0.69)

0.200 (5.08)

0.125 (3.18)

0.019 (0.48)

0.230 (5.84)

BSC

0.016 (0.41)

0.021 (0.53)

0.016 (0.41)

BASE & SEATING PLANE

0.015 (0.38)

0.008 (0.20)

0.040 (1.02) MAX

36° BSC

SEATING

0.070 (1.78)

0.100

(2.54)

BSC

0.023 (0.58)

0.014 (0.36)

PLANE

0.045 (1.14)

0.010 (0.25)

0.030 (0.76)

–8–

REV. B

AD636KCHIP [ADI]

相关型号：

AD636KCWE

AD636KD

AD636KDZ

AD636KH

AD636KHZ

AD636KN

AD636KQ

AD636_13

AD637

AD637-EVALZ

AD637A

AD637AQ