Teledyne 514 Instruction Manual page 21

light output and, therefore, the resistance of its shunt resistors. This enables
the signal at TP2 to be continually adjusted up or down to hold the reference
signal at a constant level (nominally 9 volts) and thus eliminate the effects of
turbidity or other foreign substances in the sample, within design limits.
After the automatic gain control circuit, the signal proceeds to the peak
level detector, where it is demodulated by A1 and A2, using the timing
signals previously generated (see dwg. B-14554). Potentiometer R3 can be
used to precisely balance the signal levels by adjusting the feedback loop
gain resistance of the two respective peaks. The separate peaks are then sent
through peak detector networks where they are transformed into stable DC
voltage levels. In the case of the reference peak level, it is from here that a +9
VDC signal is fed back to the automatic gain control network. The signal
levels are then fed to either one of two logarithmic ratio amplifiers (see dwgs.
C-14586 and C-14907).
For applications of high sensitivity, a chopper-stabilized log amplifier is
used (see dwgs. C-14586 and C-17706). A3 generates an approximate 200
Hz square wave which alternately allows the signals to be fed into the log
amplifier (A1) itself, and then blocks the measuring level and feeds the
reference level into both log amplifier inputs, allowing it to zero itself. The
log signal is then applied through amplifier A4 and A6 to A7 where a
coarse zero offset voltage may be applied through the ZERO potentiometer
on the power module
For less sensitive applications, a simpler log ratio circuit is used (dwg.
C-14907). The reference and measuring levels are processed through a
filter network before being compared by A1. This comparison results in
the log ratio output which is fed to A2 for application to a zero offset
voltage from the zero adjust potentiometer on the power module.
From the log amplifier the signal is finally sent to the voltage-to-
current (E-to-I) converter for transmission to the control unit. Conversion
of the voltage signal to a current signal allows for signal transmission over
greater distances without noise pickup.
The E-to-I converter (see dwg. B-14075) is set with a nominal offset
so that with 0 VDC input, 10 mA output is obtained. This baseline setting
is adjustable through R7, the zero adjustment.
The converter is scaled so that with a 0.5 VDC input, the output will
be 20 mA (set with balance potentiometer R12). Output nominally ranges
from 10 mA to 18 mA with a 0 to 0.4 VDC input. When required, zero
drift can be accommodated; i.e., inputs ranging from -0.5 to +0.5 VDC will
produce 0 to 20 mA outputs.
Teledyne Analytical Instruments
A Business Unit of Teledyne Electronic Technologies
Operational Theory 2.0
2–9