VDISP = VRVU) - VSUPPRESS kT/q (I* IREAD&
2.10.5 Non-Decade Current Gains
- In kUPPRESS&)
= kT/q(ln(
IREAD
h’PRESS
)I
= 0.026/1(‘“( IREAD
h’PRESS
)I @ 25°C
The Model 617 electrometer input uses internal decade
resistance feedback networks for the current ranges. In some
applications, non-decade current gains may be desirable. As
shown in Figure 2-20, an external feedback resistor. R,,, can
be used to serve this purpose. Limitations on the magnitude
of the feedback curr,ent require that the value of RFB be
greater than loan.
NOTE
The circuit topology of Figure 2-19 works for
positive input currents only. For bipolar input
signals, an external offset bias must be applied or
use a PNP at QL.
Note that external feedback can be temporarily calibrated
i12% using the calibration program with the calibration
jumper in the disable position. See Section 7.
:“RRENT
\ . .
IOMR
I r? I
TO RANGING
NOTE: PRESS SHIFT VOLTS TO ENTER
EXTERNAL FEEDBACK MODE
Figure 2-19. “Transdiode” Logarithmic Current Configuration
2-25
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