AD600/AD602
INPUT
1V RMS
MAX
C1LO
C1HI
C1LO
C1HI
1
1
2
16
15
16
15
(SINE WAVE)
R3
A1CM
A1OP
VPOS
VNEG
A2OP
A2CM
C2HI
A1CM
A1OP
VPOS
VNEG
A1HI
2
A1HI
R2
C4
C1
200Ω
487Ω
V
A1
A1
OUT
A1LO
3
A1LO
2µF
0.1µF
U3A
1/4
14
13
12
11
10
14
13
12
11
10
3
4
GAT1
4
GAT1
GAT2
A2LO
A2HI
+5V DEC
–5V DEC
+5V DEC
–5V DEC
R1
AD713
REF
A2
REF
A2
GAT2
5
133kΩ
5
6
C2
R5
A2LO
6
A2OP
A2CM
0.1µF
1.58kΩ
A2HI
7
7
8
U3B
1/4
R4
C3
C2HI
C2LO
8
C2LO
9
9
133kΩ
220pF
U1 AD600
U2 AD600
AD713
+5V
FB
–2dB
+2dB
–62.5mV
0dB
+62.5mV
0.1µF
0.1µF
–5V
+5V
+5V DEC
R6
R7
R8
R9
10kΩ
127Ω
127Ω
10kΩ
–5V DEC
FB
C5
22µF
–5V
POWER SUPPLY
+5V DEC
DECOUPLING NETWORK
1
14
VINP
VPOS
+5V DEC
U4
AD636
NC
13
2
3
NC
R11
R15
46.4kΩ
12 NC
11 NC
VNEG
–5V DEC
19.6kΩ
C6
R10
4
5
6
CAVG
VLOG
4.7µF
3.16kΩ
R16
COMM 10
NC
NC
Q1
6.65kΩ
2N3906
R14
9
8
BFOP
BFIN
LDLO
301kΩ
V
7
U3C
1/4
V
RMS
LOG
+316.2mV
AD713
R12
11.3kΩ
R13
3.01kΩ
NC = NO CONNECT
Figure 25. RMS Responding AGC Circuit with 100 dB Dynamic Range
100 dB to 120 dB RMS Responding Constant Bandwidth AGC
Systems with High Accuracy dB Outputs
remarkably low ±0.25 dB over the 108 dB range from 6 µV to
1.5 V rms. However, with the gain offsets connected, the gain
linearity remains under ±0.1 dB over the specified 100 dB range
(Figure 28).
The next two applications double as both AGC amplifiers and
measurement systems. In both, precise gain offsets are used to
achieve either (1) a very high gain linearity of ±0.1 dB over the
full 100 dB range, or (2) the optimal signal-to-noise ratio at any
gain.
5
4
3
A 100 dB RMS/AGC System with Minimal Gain Error
(Parallel Gain with Offset)
2
Figure 25 shows an rms-responding AGC circuit, which can
equally well be used as an accurate measurement system. It
accepts inputs of 10 µV to 1 V rms (–100 dBV to 0 dBV) with
generous overrange. Figure 26 shows the logarithmic output,
1
0
–1
–2
–3
–4
–5
V
LOG, which is accurately scaled 1 V per decade, that is,
50 mV/dB, with an intercept (VLOG = 0) at 3.16 mV rms
(–50 dBV). Gain offsets of ±2 dB have been introduced between
the amplifiers, provided by the ±62.5 mV introduced by R6–R9.
These offsets cancel a small gain ripple which arises in the
X-AMP from its finite interpolation error, which has a period of
18 dB in the individual VCA sections. The gain ripple of all
three amplifier sections without this offset (in which case the
gain errors simply add) is shown in Figure 27; it is still a
1µV
10µV
100µV
1mV
10mV 100mV
1V
10V
INPUT SIGNAL – V RMS
Figure 26. VLOG Plotted vs. VIN for Figure 25‘s Circuit
Showing 120 dB AGC Range
–14–
REV. A
ADI [ ADI ]
