參數(shù)資料
型號: AD600ARZ-R7
廠商: ANALOG DEVICES INC
元件分類: 模擬信號調(diào)理
英文描述: Dual, Low Noise, Wideband Variable Gain Amplifiers
中文描述: SPECIALTY ANALOG CIRCUIT, PDSO16
封裝: LEAD FREE, MS-013AA, SOIC-16
文件頁數(shù): 20/28頁
文件大小: 432K
代理商: AD600ARZ-R7
AD600/AD602
Rev. E | Page 20 of 28
C1HI
A1CM
A1OP
VPOS
VNEG
A2OP
A2CM
C2HI
C1LO
A1HI
A1LO
GAT1
GAT2
A2LO
A2HI
C2LO
1
2
3
4
5
6
7
14
13
12
11
10
9
8
U2
AD636
VINP
VNEG
CAVG
VLOG
BFOP
BFIN
VPOS
COMM
LDLO
V
RMS
INPUT
1V rms
MAX
(SINEWAVE)
R2 200
R3
133k
U3A
1/2
AD712
R4
3.01k
R5
16.2k
C1
0.1μF
C2
2μF
NC
NC
NC
NC
NC
NC
V
rms
AF/RF
OUTPUT
C4
4.7μF
+6V DEC
R7
56.2k
R6
3.16k
C3
1μF
U3B
1/2
AD712
+316.2mV
VOUT
+100mV/dB
0V = 0dB (AT 10mV rms)
NC = NO CONNECT
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
REF
A1
A2
+
+
U1
AD600
FB
FB
+6V
–6V
+6V
DEC
–6V
DEC
0.1μF
0.1μF
POWER SUPPLY
DECOUPLING
NETWORK
CAL
0dB
+6V
DEC
–6V
DEC
–6V
DEC
R1
115
V
G
15.625mV/dB
0
Figure 41. The Output of This Three-IC Circuit Is Proportional to the Decibel Value of the rms Input
The output of A2 is ac-coupled via another 12 Hz high-pass
filter formed by C2 and the 6.7 kΩ input resistance of the
AD636. The averaging time constant for the rms-dc converter
is determined by C4. The unbuffered output of the AD636 (at
Pin 8) is compared with a fixed voltage of 316 mV set by the
positive supply voltage of 6 V and Resistors R6 and R7. V
REF
is
proportional to this voltage, and systems requiring greater
calibration accuracy should replace the supply dependent
reference with a more stable source.
Any difference in these voltages is integrated by the op amp
U3B, with a time constant of 3 ms formed by the parallel sum
of R6/R7 and C3. Now, if the output of the AD600 is too high,
V rms is greater than the setpoint of 316 mV, causing the output
of U3B—that is, V
OUT
—to ramp up (note that the integrator is
noninverting). A fraction of V
OUT
is connected to the inverting
gain-control inputs of the AD600, so causing the gain to be
reduced, as required, until V rms is exactly equal to 316 mV, at
which time the ac voltage at the output of A2 is forced to be
exactly 316 mV rms. This fraction is set by R4 and R5 such that
a 15.625 mV change in the control voltages of A1 and A2—
which would change the gain of the cascaded amplifiers by
1 dB—requires a change of 100 mV at V
OUT
. Notice here that
since A2 is forced to operate at an output level well below its
capacity, waveforms of high crest factor can be tolerated
throughout the amplifier.
To check the operation, assume an input of 10 mV rms is
applied to the input, which results in a voltage of 3.16 mV rms
at the input to A1, due to the 10 dB loss in the attenuator. If the
system operates as claimed, V
OUT
(and hence V
G
) should be 0.
This being the case, the gain of both A1 and A2 is 20 dB and the
output of the AD600 is therefore 100 times (40 dB) greater than
its input, which evaluates to 316 mV rms, the input required at
the
AD636
to balance the loop. Finally, note that unlike most
AGC circuits that need strong temperature compensation for
the internal kT/q scaling, these voltages, and thus the output of
this measurement system, are temperature stable, arising
directly from the fundamental and exact exponential
attenuation of the ladder networks in the AD600.
Typical results are presented for a sine wave input at 100 kHz.
Figure 42 shows that the output is held close to the setpoint of
316 mV rms over an input range in excess of 80 dB.
450
300
150
10μ
100μ
10
1
100m
10m
1m
INPUT SIGNAL (V rms)
225
375
350
200
275
425
325
175
250
400
V
O
0
Figure 42. RMS Output of A2 Held Close to the Setpoint 316 mV
for an Input Range of over 80 dB
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