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AD737JRZ-5-R7 Datasheet(PDF) 15 Page - Analog Devices

Part No. AD737JRZ-5-R7
Description  Low Cost, Low Power, True RMS-to-DC Converter
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD737JRZ-5-R7 Datasheet(HTML) 15 Page - Analog Devices

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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 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
8
2.5
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
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
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.


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