AD8362
Table 5. Suggested Values for Use in Scheme of Figure 63
4
3
2
1
R1 (Ω)
R2 (Ω)
VSLP' (V/decade)
VZ' (mV)
0.334
0.336
0.333
0.340
0.355
0.380
KC (m)
11.3
10.4
6.5
O/C
S/C
0.868
1904
1346
872
480
200
96
654
1128
1520
1800
0.870
0.890
0.914
0.942
0
3.7
1.5
0.5
30
20
10
0.972
0
HIGH SLOPE DETECTORS CENTERED ON A
NARROW WINDOW
–10
–20
–30
The situation often arises in system monitoring in which an
input signal varies by much less than 60 dB, and the highest
possible sensitivity and accuracy of measurement is required
within a narrow window of input magnitudes. Adapting the
AD8362 to this task requires that the slope be increased and the
intercept repositioned. Using an attenuator from VOUT to
VSET, any slope >50 mV/dB can be realized. Then, using a
fraction of VREF (or external reference voltage), the particular
region of the dynamic range to be measured can be positioned
wherever desired. In these high slope applications, the full rail-
to-rail output swing of the AD8362 can be exploited.
100µ
1m
10m
0.1
1
10
RMS INPUT VOLTAGE (V)
Figure 66. Illustrative Results for Slope of 100 mV/dB
4
3
2
1
0
30
SLOPE
R1
R2
AD8362
(V/dec)
2
4
5
(kΩ)
4.02
8.66
8.66
9.1
(kΩ)
4.32
3.01
2.15
1.02
20
10
1
2
3
4
5
16
15
14
13
12
11
10
9
COMM ACOM
CHPF
DECL
INHI
VREF
VTGT
VPOS
VOUT
VSET
0
10
–10
–20
–30
V
INLO
DECL
OUT
R1
R2
100µ
1m
10m
0.1
1
10
6
7
8
RMS INPUT VOLTAGE (V)
PWDN ACOM
COMM CLPF
INTERCEPT OFFSET
VOLTAGE, V
Figure 67. Illustrative Results for Slope of 200 mV/dB
SHIFT
Figure 65. Slope and Intercept Adjustment
4
3
2
1
Figure 65 shows the basic connections for this mode of use; the
intercept repositioning voltage VSHIFT can be introduced by
adding a third resistor from VREF to VSET with recalculated
values of R1 and R2 or by using an external voltage source.
Figure 66 presents the simulation results for a log slope of
100 mV/dB (2 V/dec) covering two-decade spans over several
sub-ranges, while Figure 67 shows the results for a slope of
200 mV/dB (4 V/dec), providing just a one-decade span.
0
30
20
10
0
To accurately reposition the range (intercept) when very high
slopes are used, a low output impedance DAC can be used to
provide VSHIFT. Figure 68 shows simulated results for a slope of
500 mV/dB (10 V/dec) presuming this configuration. In all
cases, the fixed-pattern ripple in the log conformance remains
unchanged in dB terms. Residual fluctuations due to insufficient
averaging (in low frequency applications) are likewise
unaffected in their equivalent decibel value, though greater in
absolute voltage terms.
–10
–20
–30
100µ
1m
10m
0.1
1
10
RMS INPUT VOLTAGE (V)
Figure 68. Illustrative Results for Slope of 10 mV/dec
Rev. B | Page 31 of 36
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