Project #4 Sweeping the
HP8620c and HP8620a with your PC
Back in the late eighties
or early nineties I built a PC ISA card published by the Dutch Elektuur magazine in May 1988 to control my swept
measurements.
This card has an eight channel
13 bit ICL7109 ADC and a single channel PM7548 12 bit DA output with an output
resistance of 10 kΩ and an open voltage from
-4.096 to 4.096 Volt. The ADC is pretty slow though, about 120 ms, so you
should not be in a hurry. The board also hosts a 8255
parallel IO and a 8253 counter, but I never used the latter one.
The sweep-speed of the
circuit below operates OK at least up to 1 kHz.. At 10
kHz the op-amps appear to be too slow.
The HP8620c needs an
external sweep-voltage from 0 up to 10 Volts. The HP8620a needs a voltage of 0 ..6 Volts which must be able to sink current
!!
With the circuit described
below, the HP8620a can be driven only, if the circuit is powered by an active
HP8620c or optional power-supply.
Adjustments:
Measure on the output of the
upper left amplifier with open input and select a SOT on the resistor to -10V
such that the output is 4.096V
Check that with DAC drive
of -4.096V with 10 kΩ source resistance the
voltage becomes 0 Volt or adjust with SOT parallel to the input.
Check that with DAC drive
of +4.096V with 10 kΩ source resistance the
voltage becomes 8.192 Volt
Now check that the drive to
the HP8620c is 10 Volts, may need a SOT parallel to 2k26.