AD734
Rev. E | Page 16 of 20
is sufficient gain to raise the amplitude of E
IN
to that required to
establish an output amplitude of E
C
over the range of 1 V to 10 V.
The X input of the AD734, which has finite offset voltage, can be
troublesome at the output at high gains. The output offset is
reduced to that of the X input (1 mV or 2 mV) by the offset
loop comprising R3, C3, and Buffer A1. The low-pass corner
frequency of 0.16 Hz is transformed to a high-pass corner that is
multiplied by the gain (for example, 160 Hz at a gain of 1000).
In applications not requiring operation down to low frequencies,
Amplifier A1 can be eliminated, but the AD734’s input resistance
of 50 kΩ between X1 and X2 reduces the time constant and
increases the input offset. Using a nonpolar 20 mF tantalum
capacitor for C1 results in the same unity-gain high-pass corner; in
this case, the offset gain increases to 20, which is still acceptable.
Figure 33 shows the error in the output for sinusoidal inputs at
100 Hz, 100 kHz, and 1 MHz, with E
C
set to 10 V. The output
error for any frequency between 300 Hz and 300 kHz is similar
to that for 100 kHz. At low signal frequencies and low input
amplitudes, the dynamics of the control loop determine the gain
error and distortion; at high frequencies, the 200 MHz gain-
bandwidth product of the AD734 limits the available gain.
The output amplitude tracks E
C
over the range of 1 V to slightly
more than 10 V.
INPUT AMPLITUDE (V)
ERROR (dB)
2
1
0
–1
–2
0.01 0.1 1 10
00827-016
100kHz
100Hz
1MHz
Figure 33. AGC Amplifier Output Error vs. Input Voltage
WIDEBAND RMS-TO-DC CONVERTER USING U
INTERFACE
The AD734 is well-suited to such applications as implicit rms-
to-dc conversion, where the AD734 implements the function
[]
RMS
IN
RMS
V
V
V
2
avg
= (13)
using its direct divide mode. Figure 34 shows the circuit.
AD734
X11
X22
U03
U14
U2
5
VP 14
DD 13
W 12
Z1 11
Z2
10
Y2
7
ER
9
VN
8
Y16
+15
U2b
1/2
AD708
1/2
AD708
–15V
0.1µF
0.1µF
00827-017
L
L
L
L
L
C1
47µF
C2
1µF
V
IN
R1
3.32kΩ
U2a
L
L
V
O
= V
IN
2
Figure 34. A Two-Chip, Wideband RMS-to-DC Converter
In this application, the AD734 and an AD708 dual op amp
serve as a two-chip rms-to-dc converter with a 10 MHz
bandwidth. Figure 35 shows the circuit’s performance for
square-, sine-, and triangle-wave inputs. The circuit accepts
signals as high as 10 V p-p with a crest factor of 1 or 1 V p-p
with a crest factor of 10. The circuit’s response is flat to 10 MHz
with an input of 10 V, flat to almost 5 MHz for an input of 1 V,
and to almost 1 MHz for inputs of 100 mV. For accurate
measurements of input levels below 100 mV, the AD734’s
output offset (Z interface) voltage, which contributes a dc error,
must be trimmed out.
In the circuit shown in Figure 34, the AD734 squares the input
signal, and its output (V
IN
2
) is averaged by a low-pass filter that
consists of R1 and C1 and has a corner frequency of 1 Hz. Because
of the implicit feedback loop, this value is both the output value,
V
RMS
, and the denominator in Equation 13. U2a and U2b, an
AD708 dual dc precision op amp, serve as unity-gain buffers,
supplying both the output voltage and driving the U interface.
INPUT FREQUENCY (Hz)
OUTPUT VOLTAGE (V)
100
10
1
100m
1m
10m
100µ
10k 100k 1M 10M
00827-018
SQUARE WAVE
SINE WAVE
TRI-WAVE
Figure 35. RMS-to-DC Converter Performance
