AD640 Analog Devices, AD640 Datasheet - Page 16

• Manufacturer Part Number: AD640
• Description: DC-Coupled Demodulating 120 MHz Logarithmic Amplifier
• Manufacturer: Analog Devices

AD640
be increased and U3 can be replaced by a low speed op amp.
Figure 31 shows typical performance of this converter.
10 Hz–100 kHz Converter with 95 dB Dynamic Range
To increase the dynamic range it is necessary to reduce the
bandwidth by the inclusion of a low-pass filter at the signal
interface between U1 and U2 (Figure 32). To provide operation
down to low frequencies, dc coupling is used at the interface
between AD640s and the input offset is nulled by a feedback
circuit.
Using values of 0.02 F in the interstage filter formed by capaci-
tors C1 and C2, the hf corner occurs at about 100 kHz. U3
(AD712) forms a 4-pole 35 Hz low-pass filter. This provides
operation to signal frequencies below 20 Hz. The filter response
is not critical, allowing the use of an electrolytic capacitor to
form one of the poles.
R1 is restricted to 50
needs to be large to form a 5 ms time constant. A tantalum
capacitor is used (note polarity). The output of U3a is scaled
+1 V per decade, and the X2 gain of U3b raises this to +2 V per
decade, or +100 mV/dB. The differential offset at the output of
U2 is low-pass filtered by R6/C7 and R7/C8 and buffered by
voltage followers U4a and U4b. The 16s open loop time constant
translates to a closed loop high-pass corner of 10 Hz. (This
0.320 (8.13)
0.300 (7.62)
SEATING
PLANE
(2.41)
0.095
0.020 (0.51)
0.015 (0.38)
20-Terminal Ceramic LCC (E) Package
(2.085
0.082
20-Lead Ceramic DIP (D) Package
20
1
PIN 1
0.018
0.455)
1.010 (25.65)
0.990 (25.15)
0.430 (10.16)
(1.27)
0.050
(2.54)
0.10
by the compliance at Pin 14, so C3
0.350
13
(8.89
0.054 (1.37)
0.040 (1.01)
9
14
BOTTOM VIEW
8
INDEX
PIN 1
11
10
0.008
0.20)
0.210 (5.33)
0.150 (3.81)
1
0.300 (7.62)
0.280 (7.11)
0.085
(2.16)
18
20
4
SQ
19
3
0.040
(1.02
REF 3 PLCS
0.20
(0.51
REF
0.025
(0.635
0.300 (7.62)
45
45 )
45 )
45
0.003
0.075)
Dimensions shown in inches and (mm).
0.012 (0.30)
0.008 (0.20)
OUTLINE DIMENSIONS
–16–
high-pass filter is only operative for very small inputs; see page
13.) Figure 33 shows the performance for square wave inputs.
Since the attenuator is used, the upper end of the dynamic
range now extends to +6 dBV and the intercept is at –82 dBV.
The noise limited dynamic range is over 100 dB, but in practice
spurious signals at the input will determine the achievable range.
Figure 33. Logarithmic Output and Nonlinearity for Circuit
of Figure 32, for a Square Wave Input at f = 10 kHz
(1.143
0.125 (3.18)
0.045
–1
31.6
9
8
7
6
5
4
3
2
1
0
–90
(7.874)
0.020 (0.51)
0.310
TYP
MAX
MIN
0.076)
0.003
0.021 (0.533)
0.015 (0.381)
100
–80
(0.51)
0.020
MAX
20-Lead Plastic DIP (N) Package
20
316
1
–70
20-Lead PLCC (P) Package
(9.905
(8.966
PIN 1
4
8
0.390
0.353
1.070 (27.18)
3
(PINS DOWN)
9
TOP VIEW
IDENTIFIER
INPUT AMPLITUDE AT 10kHz
–60
1m
0.100
(2.54)
TYP
PIN 1
0.005
0.125)
0.003
0.076)
3.16m
–50
19
13
(0.838)
0.033
SQ
SQ
TYP
18
14
10m
–40
10
11
0.050
(1.27)
0.045 (1.143)
0.025 (0.635)
31.6m
SEATING
PLANE
–30
0.250
(6.350)
TYP
(4.385
0.060 (1.53) MIN
0.173
100m
–20
0.008
0.185)
15
316m
0
0.300 (7.62)
–10
0.105
(2.665
TYP
0.029
(0.737
0.017
(0.432
0.025 (0.64) MIN
0.020 (0.51) MIN
R
0.014 (0.356)
0.008 (0.203)
0.035
(0.89
0
1
0.015
0.375)
0.003
0.004
3.16
0.076)
0.180
(4.572)
MAX
0.101)
10
0.25)
0.01
–2
2
0
REV. C
dBV
V