AD637JRZ-R7 datasheet(13/25 Pages) AD

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Data Sheet

AD637

APPLICATIONS INFORMATION

STANDARD CONNECTION

The AD637 is simple to connect for a majority of rms

measurements. In the standard rms connection shown in Figure 5,

only a single external capacitor is required to set the averaging

time constant. In this configuration, the AD637 computes the

true rms of any input signal. An averaging error, the magnitude

of which is dependent on the value of the averaging capacitor,

is present at low frequencies. For example, if the filter capacitor,

CAV, is 4 µF, the error is 0.1% at 10 Hz and increases to 1% at

3 Hz. To measure ac signals, the AD637 can be ac-coupled by

adding a nonpolar capacitor in series with the input, as shown

in Figure 5.

25kΩ

DEN

INPUT

CAV

BUFF IN

BUFF OUT

SQUARER/

DIVIDER

OUTPUT

3 COMMON

ABSOLUTE

VALUE

BIAS

2 NIC

OUTPUT

OFFSET

4.7kΩ

+VS

25kΩ

NIC

–VS

+VS

VIN

VOUT = V

(OPTIONAL)

VIN

CAV

AD637

RMS

OUT

–

NIC = NO INTERNAL CONNECTION

Figure 5. Standard RMS Connection

The performance of the AD637 is tolerant of minor variations

in the power supply voltages; however, if the supplies used

exhibit a considerable amount of high frequency ripple, it is

advisable to bypass both supplies to ground through a 0.1 µF

ceramic disc capacitor placed as close to the device as possible.

The output signal range of the AD637 is a function of the

supply voltages, as shown in Figure 6. Use the output voltage

buffered or nonbuffered, depending on the characteristics of the

load. Connect the buffer input (Pin 1) to common if buffering is

not required. The output of the AD637 is capable of driving

5 mA into a 2 kΩ load without degrading the accuracy of the

device.

SUPPLY VOLTAGE – DUAL SUPPLY (V)

±3

±5

±10

±15

±18

Figure 6. Maximum VOUT vs. Supply Voltage

CHIP SELECT

The AD637 includes a chip select feature that allows the user

to decrease the quiescent current of the device from 2.2 mA to

350 µA. This is done by driving CS, Pin 5, to below 0.2 V dc.

Under these conditions, the output goes into a high impedance

state. In addition to reducing the power consumption, the

outputs of multiple devices can be connected in parallel to

form a wide bandwidth rms multiplexer. Tie Pin 5 high to

disable the chip select.

OPTIONAL TRIMS FOR HIGH ACCURACY

The AD637 includes provisions for trimming out output offset

and scale factor errors resulting in significant reduction in the

maximum total error, as shown in Figure 7. The residual error is

due to a fixed input offset in the absolute value circuit and the

residual nonlinearity of the device.

5.0

2.5

–5.0

2.0

0.5

1.0

–2.5

1.5

AD637K MAX

INTERNAL TRIM

AD637K

EXTERNAL TRIM

AD637K: 0.5mV ± 0.2%

0.25mV ± 0.05%

EXTERNAL

INPUT LEVEL (V)

Figure 7. Maximum Total Error vs.

Input Level AD637K Internal and External Trims

Rev. L | Page 13 of 25

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