Analog Devices AD602 - Manual (Page 11)

AD600/AD602
REV. A –11–
The emitter circuit of Q1 is somewhat inductive (due its finite f
t
and base resistance). Consequently, the effective value of R2 in-
creases with frequency. This would result in an increase in the
stabilized output amplitude at high frequencies, but for the ad-
dition of C3, determined experimentally to be 15 pF for the
2N3904 for maximum response flatness. Alternatively, a faster
transistor can be used here to reduce HF peaking. Figure 16
shows the ac response at the stabilized output level of about
1.3 V rms. Figure 17 demonstrates the output stabilization for
sine wave inputs of 1 mV to 1 V rms at frequencies of 100 kHz,
1 MHz and 10 MHz
FREQUENCY – MHz
AGC OUTPUT CHANGE – dB
1 100 10
3dB
0.1
Figure 16. AC Response at the Stabilized Output Level
of 1.3 V RMS
0.001 0.01 1 0.1
INPUT AMPLITUDE – Volts RMS
RELATIVE OUTPUT – dB –0.4
+0.2
–0.2
0
100kHz
1MHz
10MHz
Figure 17. Output Stabilization vs. RMS Input for
Sine Wave Inputs at 100 kHz, 1 MHz, and 10 MHz
While the “bandgap” principle used here sets the output ampli-
tude to 1.2 V (for the square wave case), the stabilization point
can be set to any higher amplitude, up to the maximum output
of
±
(V
S
– 2) V which the AD600 can support. It is only neces-
sary to split R2 into two components of appropriate ratio whose
parallel sum remains close to the zero-TC value of 806
Ω
. This
is illustrated in Figure 18, which shows how the output can be
raised, without altering the temperature stability.
AD590
R2A
C3
15pF
300 µ A
(at 300K)
Q1
2N3904
V PTAT
RF
OUTPUT
+5V
R2B
C2
1 µ F
R2 = R2A  R2B ≈ 806 Ω
TO AD600 PIN 16
TO AD600 PIN 11
Figure 18. Modification in Detector to Raise Output to
2 V RMS
A Wide Range, RMS-Linear dB Measurement System
(2 MHz AGC Amplifier with RMS Detector)
Monolithic rms-dc converters provide an inexpensive means to
measure the rms value of a signal of arbitrary waveform, and
they also may provide a low accuracy logarithmic (“decibel-
scaled”) output. However, they have certain shortcomings. The
first of these is their restricted dynamic range, typically only
50 dB. More troublesome is that the bandwidth is roughly pro-
portional to the signal level; for example, the AD636 provides a
3 dB bandwidth of 900 kHz for an input of 100 mV rms, but
has a bandwidth of only 100 kHz for a 10 mV rms input. Its
logarithmic output is unbuffered, uncalibrated and not stable
over temperature; considerable support circuitry, including at
least two adjustments and a special high TC resistor, is required
to provide a useful output.
All of these problems can be eliminated using an AD636 as
merely the detector element in an AGC loop, in which the differ-
ence between the rms output of the amplifier and a fixed dc ref-
erence are nulled in a loop integrator. The dynamic range and
the accuracy with which the signal can be determined are now
entirely dependent on the amplifier used in the AGC system.
Since the input to the rms-dc converter is forced to a constant
amplitude, close to its maximum input capability, the band-
width is no longer signal dependent. If the amplifier has an ex-
actly exponential (“linear-dB”) gain-control law, its control
voltage VG is forced by the AGC loop to be have the general
form:
V
OUT
=
V
SCALE
log 10
V
IN (RMS )
V
REF
(4)
Figure 19 shows a practical wide dynamic range rms-responding
measurement system using the AD600. Note that the signal out-
put of this system is available at A2OP, and the circuit can be
used as a wideband AGC amplifier with an rms-responding de-
tector. This circuit can handle inputs from 100
µ
V to 1 V rms
with a constant measurement bandwidth of 20 Hz to 2 MHz,
limited primarily by the AD636 rms converter. Its logarithmic
output is a loadable voltage, accurately calibrated to 100 mV/dB,
or 2 V per decade, which simplifies the interpretation of the
reading when using a DVM, and is arranged to be –4 V for an
input of 100
µ
V rms input, zero for 10 mV, and +4 V for a
1 V rms input. In terms of Equation 4, V
REF
is 10 mV and
V
SCALE
is 2 V.