AD600/AD602
These problems can be eliminated using an
as the
detector element in an AGC loop, in which the difference
between the rms output of the amplifier and a fixed dc reference
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 bandwidth is
no longer signal dependent. If the amplifier has an exactly
exponential (linear-dB) gain-control law, its control voltage V
G
is forced by the AGC loop to have the general form
V
OUT
=
V
SCALE
log 10
V
IN
(
RMS
)
V
REF
RELATIVE OUTPUT (dB)
+0.2
100kHz
0
1MHz
–0.2
–0.4
10MHz
0.001
0.01
0.1
INPUT AMPLITUDE (V rms)
1
00538-037
(4)
Figure 39. Output Stabilization vs. rms Input for
Sine Wave Inputs at 100 kHz, 1 MHz, and 10 MHz
While the band gap principle used here sets the output
amplitude 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 that the AD600 can support. It is only
necessary to split R2 into two components of appropriate ratio
whose parallel sum remains close to the zero-TC value of
806 Ω. Figure 40 shows this and how the output can be raised
without altering the temperature stability.
5V
AD590
TO AD600 PIN 16
C2
1µF
300µA
(AT 300K)
responding measurement system using the AD600. Note that
the signal output of this system is available at A2OP, and the
circuit can be used as a wideband AGC amplifier with an rms-
responding detector. This circuit can handle inputs from
100 μV 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 μV 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.
Note that the peak log output of ±4 V requires the use of ±6 V
supplies for the dual op amp U3 (AD712) although lower
supplies would suffice for the AD600 and
If only ±5 V
supplies are available, it is necessary to either use a reduced
value for V
SCALE
(say 1 V, in which case the peak output would
be only ±2 V) or restrict the dynamic range of the signal to
about 60 dB.
As in the previous case, the two amplifiers of the AD600 are
used in cascade. However, the 6 dB attenuator and low-pass
filter found in Figure 21 are replaced by a unity gain buffer
amplifier U3A, whose 4 MHz bandwidth eliminates the risk of
instability at the highest gains. The buffer also allows the use of
a high impedance coupling network (C1/R3) that introduces a
high-pass corner at about 12 Hz. An input attenuator of 10 dB
(X0.316) is now provided by R1 + R2 operating in conjunction
with the AD600’s input resistance of 100 Ω. The adjustment
provides exact calibration of the logarithmic intercept V
REF
in
critical applications, but R1 and R2 can be replaced by a fixed
resistor of 215 Ω if very close calibration is not needed, because
the input resistance of the AD600 (and all other key parameters
of it and the
is already laser trimmed for accurate
operation. This attenuator allows inputs as large as ±4 V to be
accepted, that is, signals with an rms value of 1 V combined
with a crest factor of up to 4.
Q1
2N3904
R2B
C3
15pF
R2A
+
V
PTAT
–
R2 = R2A || R2B
≈
806Ω
00538-038
TO AD600 PIN 11
RF
OUTPUT
Figure 40. 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; they
can also 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 proportional
to the signal level; for example, the
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.
Rev. E | Page 19 of 28
AD [ ANALOG DEVICES ]
