AD600/AD602
Rev. F | Page 19 of 32
INPUT AMPLITUDE (V rms)
REL
TIVE OUTPUT (dB)
–0.4
+0.2
–0.2
0
0.001 0.01 0.1 1
100kHz
1MHz
10MHz
00538-037
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.
R2A
Q1
2N3904
V
PTAT
RF
OUTPUT
R2B
TO AD600 PIN 16
TO AD600 PIN 11
+
–
AD590
5
R2 = R2A || R2B ≈ 806Ω
300µA
(AT 300K)
C2
1µF
C3
15pF
00538-038
Figure 40. Modification in Detector to Raise Output to 2 V rms
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, when the AD600/AD602 are
used in conjunction with the AD636, as shown in Figure 41, 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.
These problems can be eliminated using an AD636 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.
Because 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
()
REF
rmsIN
SCALEOUT
V
V
VV 10log=
(4)
Figure 41 shows a practical wide dynamic range rms-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, 0 V 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 suffice for the AD600 and AD636. 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
(0.316×) is now provided by R1 + R2 operating in parallel with
the input resistance of 100 Ω of the AD600. 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 AD636) 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.
