Stanford Research Systems DS360 Operating Manual And Programming Reference page 127

The filters are programmed within a decade by varying the resistors on J301 and J302.
There are 9 steps per decade, set by R0 - R5 (U309). Decades are set by changing
capacitors C301 - C304 and C317 - C320 with relays K301, K302, K304, K305. The
resistor and capacitor settings are always the same for both filters.
The filters can be bypassed for certain situations by relay K303. U308 provides
additional gain before the signal passes through the attenuators.
Amplitude and Offset Control
U401, U402, U403 and U404 is the burst circuitry. When bursts are active, the dual DAC
U401 (AD7549) sets the "on" and "off" levels for bursts. U403 A and B (OP-275) are
current to voltage converters for U401. U404 (DG444) selects between the two levels,
based on a signal from the digital board. U402 (OP-275) acts as input and output buffers.
P401 removes any DC offset voltage between the "on" and "off" levels.
Relay K407 selects either square waves or all other signals. Relay K405 switches the
burst DACs in or out.
Relays K401, K402, K403, K404, K406 and their associated resistors make up a 1 kΩ
pre-attenuator chain. All or none of the attenuators may be switched in, depending on the
output amplitude.
U405 (REF02) is a low drift +5.00 V
to reduce its output noise. DAC U406 (AD7542) and U407 (NE5532) set the offset
voltage from -5.00 to +5.00 V
Output Amplifiers
There are two independent output amplifiers. Each has a separate pre-amplifier. U501
(AD797) provides a gain of +1.6 to the upper amplifier. U504 (AD797) provides a gain
of -1.6 to the lower amplifier. P506 is used to balance the upper and lower amplifiers.
U502 and U505 (NE5534) provide a low frequency DC stabilization loop for the two
output amplifiers. They adjust the non-inverting inputs of the output amplifiers to
maintain the correct DC voltage at the outputs. P503 balances the DC output voltage
between the two.
Discrete Amplifiers
The two amplifiers are identical. For simplicity's sake, only the upper amplifier will be
described.
The output amplifiers are essentially discrete op-amps. U503-5 is the inverting input,
U503-2 is the non-inverting input and the junction between R527 and R528 is the output.
The amplifiers are connected in an inverting configuration, with R507 the input resistor,
and R511 and P501 the feedback resistor. The non-inverting input is grounded through
R515. C503 provides feedback compensation, to prevent oscillations.
U503 (MAT-02) is a dual, low noise transistor, that is used as a differential amplifier.
Q505 is a current source, which draws a constant 1.6 mA (0.8 mA through each leg of
U503). Q503, Q504 and Q506 are configured as a cascode, to maintain a constant voltage
across U503. Q501 and Q502 are active collector loads for U503 A and B. Q507 provides
a low impedance source to Q508, which provides most of the voltage gain. It is biased at
3.5 mA by Q509.
reference. Its output is filtered by L401 and C404
DC
.
DC
DS360 Ultra Low Distortion Function Generator
Circuit Description 7-7