We will now define V
T
(the corrected observed temperature-induced shift of the main output at the
start of logging) to be the mean of the absolute values of the hot and cold shifts, corrected as
described above. The outputs of A1 and A2 should not shift over temperature by more than about
2 mV and 4 mV, respectively. It is almost always possible to correct the temperature-induced low-
power shift of the output, V
T
, by simply inducing an opposite temperature- induced shift to
compensate for the previously-existing one. This is accomplished by offsetting A2
in
- (pin 31),
using RT2.
If we assume that the corrected observed low-power shift is V
T
(as above), the gain setting of A3 is
G3, and the overall output slope is S mV/dB, then V2, the required compensating offset voltage at
the output of A2, is given by:
V
T
x 50 mV/dB
S mV/dB
V2 =
G3 x 6.5
For example, if V
T
= 120 mV, S = 40 mV/dB, and G3 = 16, then:
120 mV x 50/40
V2 =
= 1.44 mV.
16 x 6.5
Measurement has shown that a 200k resistor (RT2) tied from the inverting input of A2 (pin 31) to
V
CC
or V
EE
will induce a 1 mV shift at the output of A2 over a 65
0
C temperature change. The
magnitude of this shift varies inversely with the value of RT2, but do not use an RT2 value of less
than 50k. If a resistor smaller than 50k is called for, you will have to offset A1 (using RT1) to
overcome the shift, as described in sub-section 8.3.4. In our example above, tying RT2 to V
CC
will
cause the output of A2 to increase as the temperature increases, while tying it to V
EE
will cause the
A2 output to decrease with increasing temperature.
Now install the appropriate value of RT2, as well as the appropriate values of RT3 and RT4 (as
selected in previously in Step Two, sub- section 8.3.2). Re-zero the output of A2 to within 100
V
of the room-temperature value it had before these adjustments. The unit should now be correctly
temperature-compensated.
8.3.4
Adjusting the Offset of A1:
If too large an offset is needed at A2 (one which would
require an RT2 value of less than 50k), it is better to offset A1 instead, using RT1. The values of
RT1 needed are gain-dependent, and need to be calibrated once for your setup. Note that the
direction of the correction is reversed here, i.e. tying RT1 to V
CC
will cause the output of A1 to
decrease as the temperature increases.
-29-
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