A2 has adequate negative swing as is. A3, however, only swings about -400 mV, and needs to
swing about -500 mV. Much less current is needed in this instance, and a resistor of about 30k
ohms from pin 26 to V
EE
will do the job. The resistance between the internal output of A3 and pin
26 is about 200 ohms, as shown in Fig. 2(b), so another balance adjustment is required, using the
procedure below:
1.
Measure the DC output level of the output amplifier.
2.
Connect the ~30k-ohm resistor from pin 26 to V
EE
.
3.
Adjust the A3 DC offset to restore the output amplifier's DC output level to its prior
(step 1) value.
Please note that after the above two re-balancing procedures, the external outputs of A1 and A3
(pins 6 and 26, respectively,) will no longer represent the actual output of these amps to the next
stage.
The logging stages are full-differential, and present no problems with negative output. The output
amplifier will swing about -2.5 V using the same current-sink approach, and in this case there is no
significant problem with ensuing temperature effects. However, since the required sink current is
equal to the negative swing divided by the load resistance (e.g., driving a 50-ohm load with a 2.0-
volt negative swing requires 40 mA), it often makes more sense to use a buffer amp at the output.
About 1.5 mA of quiescent current is available. In the case of extended-range DLVA applications
(see section 9), this same buffer could be used as the summing amplifier.
2.7 NON-LOGARITHMIC TRANSFERS
The linear extension input, the L6 and L1 current adjustments, and other user-accessible points,
can be used to generate a variety of non-logarithmic transfer curves. As one example, suppose
there is a need for a power-law type of deviation from a normal log transfer, as sketched in Fig.
2.7(a). In this instance, one might replace R14, the resistor which sets the linear extension bias
current (see Fig. 2(c)), with an appropriately-biased diode, as shown in Fig. 2.7(b).
The current through the diode D1 controls the magnitude of the transfer upswing, and can be
selected by a resistor to V
EE
. R13 determines the onset of the upswing. The diode D2 helps
offset temperature-induced effects. Please contact ANADYNE if you have specific interest in
special transfer curves.
-10A-
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