AD640
REV. C
–16–
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
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
by the compliance at Pin 14, so C3
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
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.
INPUT AMPLITUDE AT 10kHz
–1
–90
31.6
LOG
OUTPUT
FROM
CIRCUIT
OF
FIGURE
32
–
V
0
1
2
3
4
5
6
7
8
9
–80
100
–70
316
–60
1m
–50
3.16m
–40
10m
–30
31.6m
–20
100m
–10
316m
0
1
10
3.16
dBV
V
–2
0
2
ERROR
FROM
IDEAL
TRANSFER
FUNCTION
–
dB
Figure 33. Logarithmic Output and Nonlinearity for Circuit
of Figure 32, for a Square Wave Input at f = 10 kHz
C1297b–0–12/99
(rev.
C)
PRINTED
IN
U.S.A.
20-Lead Ceramic DIP (D) Package
20
1
10
11
0.300 (7.62)
0.280 (7.11)
PIN 1
0.320 (8.13)
0.300 (7.62)
0.430 (10.16)
SEATING
PLANE
0.020 (0.51)
0.015 (0.38)
1.010 (25.65)
0.990 (25.15)
0.095
(2.41)
0.210 (5.33)
0.150 (3.81)
0.054 (1.37)
0.040 (1.01)
0.10
(2.54)
0.085
(2.16)
0.012 (0.30)
0.008 (0.20)
0.300 (7.62)
20-Terminal Ceramic LCC (E) Package
1
20
4
9
8
13
19
14
3
18
PIN 1
INDEX
BOTTOM VIEW
0.20
45
(0.51
45 )
REF
0.025
0.003
(0.635
0.075)
0.050
(1.27)
0.040
45
(1.02
45 )
REF 3 PLCS
0.350
0.008
(8.89
0.20)
SQ
0.082
0.018
(2.085
0.455)
20-Lead Plastic DIP (N) Package
20
110
11
PIN 1
0.250
(6.350)
TYP
1.070 (27.18)
0.310
(7.874)
TYP
0.014 (0.356)
0.008 (0.203)
0.180
(4.572)
MAX
0.300 (7.62)
TYP
15
0
SEATING
PLANE
0.045 (1.143)
0.025 (0.635)
0.021 (0.533)
0.015 (0.381)
0.100
(2.54)
TYP
0.125 (3.18)
MIN
0.033
(0.838)
TYP
20-Lead PLCC (P) Package
0.017
0.004
(0.432
0.101)
0.029
0.003
(0.737
0.076)
0.025 (0.64) MIN
0.060 (1.53) MIN
0.020 (0.51) MIN
R
0.035
0.01
(0.89
0.25)
0.173
0.008
(4.385
0.185)
0.105
0.015
(2.665
0.375)
3
PIN 1
IDENTIFIER
4
19
18
8
9
14
13
TOP VIEW
(PINS DOWN)
0.020 (0.51)
MAX
0.050
(1.27)
0.045
0.003
(1.143
0.076)
0.353
0.003
(8.966
0.076)
SQ
0.390
0.005
(9.905
0.125)
SQ
0.020
(0.51)
MAX