AD737

Data Sheet

Rev. J | Page 16 of 24

Battery Operation

All the level-shifting for battery operation is provided by

the 3½ digit converter, shown in Figure 27. Alternatively, an

external op amp adds flexibility by accommodating nonzero

common-mode voltages and providing output scaling and offset

to zero. When an external operational amplifier is used, the

output polarity is positive going.

Figure 28 shows an op amp used in a single-supply application.

Note that the combined input resistor value (R1 + R2 + 8 kΩ)

matches that of the R5 feedback resistor. In this instance, the

magnitudes of the output dc voltage and the rms of the ac input

are equal. R3 and R4 provide current to offset the output to 0 V.

Scaling the Output Voltage

The output voltage can be scaled to the input rms voltage. For

example, assume that the AD737 is retrofitted to an existing

application using an averaging responding circuit (full-wave

rectifier). The power supply is 12 V, the input voltage is 10 V

ac, and the desired output is 6 V dc.

For convenience, use the same combined input resistance as

shown in Figure 28. Calculate the rms input current as

OUTMAG

INMAG

I

I

=

µ

=

+

+

=

A

125

kΩ

8

kΩ

2.5

kΩ

69.8

V

10

(2)

Next, using the IOUTMAG value from Equation 2, calculate the new

feedback resistor value (R5) required for 6 V output using

kΩ

48.1

μA

125

V

6

=

=

R5

(3)

Select the closest-value standard 1% resistor, 47.5 kΩ.

Because the supply is 12 V, the common-mode voltage at the

R7/R8 divider is 6 V, and the combined resistor value

(R3 + R4) is equal to the feedback resistor, or 47.5 kΩ.

R2 is used to calibrate the transfer function (gain), and R4 sets

the output voltage to zero with no input voltage.

Perform calibration as follows:

1. With no ac input applied, adjust R4 for 0 V.

2. Apply a known input to the input.

3. Adjust the R2 trimmer until the input and output match.

The op amp selected for any single-supply application must be a

rail-to-rail type, for example an AD8541, as shown in Figure 28.

For higher voltages, a higher voltage part, such as an OP196,

can be used. When calibrating to 0 V, the specified voltage

above ground for the operational amplifier must be taken into

account. Adjust R4 slightly higher as appropriate.

ADDITIONAL INFORMATION

For additional information about rms-to-dc conversions, see

the AN-268 Application Note or download the Analog Devices,

Inc., RMS to DC Conversion Application Guide.

Table 6. AD737 Capacitor Selection

Application

RMS Input Level

Low Frequency

Cutoff (−3 dB)

Maximum

Crest Factor

CAV (µF)

CF(µF)

Settling Time1 to 1%

General-Purpose RMS Computation

0 V to 1 V

20 Hz

5

150

10

360 ms

200 Hz

5

15

1

36 ms

0 mV to 200 mV

20 Hz

5

33

10

360 ms

200 Hz

5

3.3

1

36 ms

General-Purpose Average Responding

0 V to 1 V

20 Hz

None

33

1.2 sec

200 Hz

None

3.3

120 ms

0 mV to 200 mV

20 Hz

None

33

1.2 sec

200 Hz

None

3.3

120 ms

SCR Waveform Measurement

0 mV to 200 mV

50 Hz

5

100

33

1.2 sec

60 Hz

5

82

27

1.0 sec

0 mV to 100 mV

50 Hz

5

50

33

1.2 sec

60 Hz

5

47

27

1.0 sec

Audio Applications

Speech

0 mV to 200 mV

300 Hz

3

1.5

0.5

18 ms

Music

0 mV to 100 mV

20 Hz

10

100

68

2.4 sec

1

Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times are greater for decreasing amplitude input signals.