Datasheet

Data Sheet AD737
Rev. I | Page 15 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 1 2 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
II =µ=
++
= A 125
kΩ
8 kΩ 2.5 kΩ 69.8
V 10
(2)
Next, using the I
OUTMAG
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.
Table 6. AD737 Capacitor Selection
Application RMS Input Level
Low Frequency
Cutoff (−3 dB)
Maximum
Crest Factor C
AV
(µF) C
F
(µF) Settling Time
1
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.