Analog Board Description - Stanford Research Systems DS360 Operating Manual And Programming Reference

7-6 Circuit Description
Analog Board
This section covers schematics ANA-1 through ANA-7 at the end of this chapter.
The analog board contains the interface logic to the digital board, the waveform DAC, all
signal conditioning circuitry, attenuators, amplifiers and analog power supplies.
Digital - Analog Interface
U101, U102 and U103 (HCPL-7100) are low jitter opto-isolators that transmit the serial
waveform data from the DSP to the waveform DAC. The interface consists of the data,
clock for the data, and a frame sync signal to load the DAC.
U106, U107 and U108 (6N137) are opto-isolators that transmit control signals from the
main processor to the analog board. 16 bits of data is clocked into shift registers U109
and U110 (74HC164) whenever any parameter is changed on the analog board. The three
highest bits (first 3 bits clocked in) go to U111 (74HC138), which provides chip selects
for the various devices.
U112 (6N137) is used to control the burst logic. U114 (HCPL-7100) is the sync signal
that is used on the digital board.
Waveform Generation
The waveform DAC is used to generate all output waveforms. To generate sine waves,
white noise, sweeps or 2-Tones, relay K201 selects the output of the anti-imaging filter
directly. For pink noise, the waveform DAC generates white noise, which is filtered into
pink noise by U207 (LF357) and its associated resistors and capacitors. Relay K201 then
selects the output of U207. Square waves are generated by using a comparator to sense
zero crossings of a sine wave. They are switched into the circuit on ANA-4.
The waveform DAC is an Analog Devices AD1862-J, a 20 bit, ultra low distortion DAC,
capable of generating a sinewave with < -100dBc THD (total harmonic distortion). U202
(AD797) is its associated current to voltage converter. They feed a seventh order Cauer
anti-imaging filter which filters out any out of band frequency components.
The heart of the square wave and sync generator is the high speed analog comparator,
U209 (LT1016). The sine wave is buffered and amplified by U208A (LF412) to ensure
that the comparator operates over all input signal levels. Low frequency hysteresis is
provided by U210 (LM311) and U113A. High frequency hysteresis is provided by C222.
The output of the LT1016 is raised to full TTL (0 to +5 V
by relay K202, this signal is amplified by U208B (LF412) to ±6.5 V
P201 is the low frequency square wave phase adjust that sets the square wave symmetry.
C225 controls the rise time of the square wave.
Distortion Reduction Filters
There are two cascaded programmable distortion reduction filters. These operate by
attenuating the distortion products (2 * F
signal. Each filter is a second order, state variable, low pass filter. J301 and J302 are
connected to switchable resistor networks.
DS360 Ultra Low Distortion Function Generator
) by U113B. When selected
DC
3 * F ...) without affecting the fundamental
o, o
.
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