AD602 AD [Analog Devices], AD602 Datasheet - Page 14

• Manufacturer Part Number: AD602
• Description: Dual, Low Noise, Wideband Variable Gain Amplifiers
• Manufacturer: AD [Analog Devices]

AD600/AD602
100 dB to 120 dB RMS Responding Constant Bandwidth AGC
Systems with High Accuracy dB Outputs
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.
A 100 dB RMS/AGC System with Minimal Gain Error
(Parallel Gain with Offset)
Figure 25 shows an rms-responding AGC circuit, which can
equally well be used as an accurate measurement system. It
accepts inputs of 10 V to 1 V rms (–100 dBV to 0 dBV) with
generous overrange. Figure 26 shows the logarithmic output,
V
50 mV/dB, with an intercept (V
(–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
(SINE WAVE)
LOG
1V RMS
INPUT
MAX
, which is accurately scaled 1 V per decade, that is,
3.01k
R13
C1LO
A1LO
GAT1
A2LO
GAT2
C2LO
A1HI
A2HI
1
2
3
4
5
6
7
8
+5V DEC
–5V DEC
2N3906
DECOUPLING NETWORK
U1 AD600
Q1
11.3k
R12
POWER SUPPLY
FB
FB
Figure 25. RMS Responding AGC Circuit with 100 dB Dynamic Range
301k
A1
A2
+5V
–5V
REF
R14
LOG
0.1 F
0.1 F
+5V DEC
16
15
14
13
12
11
10
9
= 0) at 3.16 mV rms
C1HI
A1CM
A1OP
VPOS
VNEG
A2OP
A2CM
C2HI
R15
19.6k
R16
6.65k
+5V DEC
–5V DEC
–5V DEC
133k
0.1 F
C1
R1
NC
NC
NC
1
2
3
4
5
6
7
133k
0.1 F
U3A
C2
VINP
VNEG
CAVG
VLOG
BFOP
BFIN
R4
1/4
AD713
AD636
–5V
22 F
U4
1.58k
C5
220pF
–14–
R5
COMM
VPOS
LDLO
C3
10k
V
R6
RMS
487
remarkably low 0.25 dB over the 108 dB range from 6 V 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).
Figure 26. V
Showing 120 dB AGC Range
R2
14
13
12
11
10
9
8
–2dB
–62.5mV
127
R7
NC
NC
NC
200
R3
U3B
3.16k
+316.2mV
1/4
AD713
R10
–1
–2
–3
–4
–5
0dB
127
5
4
3
2
1
0
R8
1 V
+5V DEC
R11
46.4k
LOG
+2dB
+62.5mV
NC = NO CONNECT
10k
R9
10 V
4.7 F
U3C
C6
1/4
AD713
Plotted vs. V
A1LO
GAT1
C1LO
GAT2
A2LO
C2LO
A1HI
A2HI
+5V
100 V
INPUT SIGNAL – V RMS
1
2
3
4
5
6
7
8
U2 AD600
1mV
V
IN
LOG
A1
A2
REF
for Figure 25‘s Circuit
10mV
16
15
14
13
12
11
10
9
C1HI
A1CM
A1OP
VPOS
VNEG
A2OP
A2CM
C2HI
100mV
+5V DEC
–5V DEC
1V
2 F
C4
10V
REV. A
V
OUT