Analog Input Board - Stanford Research Systems SR810 Manual

ANALOG INPUT BOARD

OVERVIEW
The Analog Input Board provides the very
important link between the user's input signal and
the DSP processor. From the front panel BNC, the
user's signal passes through a low distortion front-
end amplifier, gain stages, notch filters, anti-
aliasing filter, and finally an A/D Converter. Once
converted to digital form, the input signal is ready
to be processed by the Digital Signal Processor.
INPUT AMPLIFIER
The goal of any measurement instrument is to
perform some given measurement while affecting
the quantities to be measured as little as possible.
As such, the input amplifier is often the most
critical stage in the entire signal path. The design
of the front end input amplifier in the SR810 was
driven by an effort
performance in the following areas: input voltage
noise, input current noise, input capacitance,
harmonic distortion, and common mode rejection
(CMR). To provide such performance, a FET input
differential amplifier with common-mode feedback
architecture was chosen. The input signal is first
passed through a series of relays to select input
mode and input coupling. The input FETs U100A
and U100B are extremely low-noise matched
FETs. To improve distortion performance, the
input FETs are cascoded to maintain a constant
drain-source voltage across each FET. This
prevents modulation of the drain-source voltage
by the input voltage. U109 senses the source
voltages and maintains the same voltage at the
drains (via FETs U108A and B) with some DC
offset determined by resistors N102 and N103.
U105 provides common-mode feedback and
maintains a constant drain current in each FET.
The gain of the front end is fixed. U103 provides
the output. The DC offset is adjusted by P101 and
the CMR by P102.
GAIN STAGES AND NOTCH FILTERS
Collectively, the front end amplifier and following
gain stages provide gain up to about 2000.
The notch filters are simple single stage, inverting
band pass filters summing with their inputs to
remove 60 Hz or 120 Hz. Each filter has a depth
and frequency adjustment. (60 Hz - depth:P222
to provide optimum
Circuit Description
and freq:P221
freq:P201). The 120 Hz notch filter has a
configurable gain of either 1 or 3.17.
The notch filters are followed by two gain stages,
each configurable up to a gain of 10.
Overloads are sensed at the input amplifier and
the final amplifier outputs. Since there is no
attenuation in the amplifier chain, this is sufficient.
ANTI-ALIASING FILTER
To prevent aliasing, the input signal passes
through a low-pass filter so that all frequency
components greater than half the sampling
frequency are attenuated by at least 96 dB. This is
accomplished with an 8-zero 9-pole elliptical low
pass filter. The pass band of this filter is DC to
102 kHz. The stopband begins at 154 kHz.
Stopband attenuation is nominally 100 dB.
The architecture of the filter is based on a singly
terminated passive LC ladder filter. L's are
simulated with active gyrators formed by op-amp
pairs (U311, U321, U331, U341). Passive LC
ladder filters have the special characteristic of
being very tolerant of variations in component
values. Because no section of the ladder is
completely isolated from the other, a change in
value of any single component affects the entire
ladder. The design of the LC ladder however, is
such that the characteristics of the rest of the
ladder will shift to account for the change in such a
way as to minimize its effect on the ladder. Not
only does this loosen the requirement for
extremely high accuracy resistors and capacitors,
but it also makes the filter extremely stable despite
wide temperature variations. As such, the anti-
aliasing filter used in the SR810 does not ever
require calibration to meets its specifications.
Following the anti-aliasing filter is the signal
monitor buffer (U386) and A/D driver stage
(U301).
A/D CONVERTER
The SR810 uses a dual channel A/D converter
(U407). Each channel samples simultaneously at
a rate of 256 kHz. One channel is dedicated to the
input signal. The other channel reads one of the
7-7
120 Hz - depth:P202 and