DIFFERENTIAL LOW-NOISE FRONT END
Two high-impedance inputs A and B allow the instrument to operate in either single- ended or true differential modes. Relays K103 and K104 allow the inputs A and B to be individually grounded, while K101 selects AC or DC coupling. Inversion of the inputs is provided by relay K105. The input capacitances and R101 and R102 establish the front end's input impedance at 25 pF and 100 MΩ.
U106 is an NPD5564 low-noise matched FET pair, which, along with U102 and U103 form the first differential amplifier stage. U102 compares the currents in the drain loads of U106, and U103 maintains the sum of those currents at a fixed level by varying the total current in both FETs. C109 provides open-loop compensation for U102, and front-end gain is nominally established by the sum of R118 and R112 over the sum of R114 and R128. K102 is a gain switching relay which selects a front end gain of 2 or
10.In the gain of 2 position, gain to the next stage becomes 1 when R116 divides with the input attenuator to the next stage. For a gain of 10, relay K102 shorts the top of R115 and R128 together, essentially eliminating them from the gain loop. P103 allows adjustment of front-end offset, and P104 allows for offset compensation when in the low gain configuration. P102 allows adjustment of the front-end common-mode rejection ratio, along with P101, which adjusts the CMRR in the low gain configuration.
In the second gain stage, U105 is configured with a fixed gain of 10. By switching the input attenuation of this stage with DG444 U101, the overall gain of this stage can be computer selected as 2, 5, or
10.C111 provides high frequency compensation for U105. The output of this stage passes through all three sections of U104, a CMOS multiplexer that serves as the blanking control. The three parallel switches provide a low "on" resistance to
select either the output of the second stage amplifier or ground as the input to the next stage, the first filter section.
CONFIGURABLE FILTERS AND GAIN
The two filter stages in the SR560 each consist of 16 R-C filters which can be configured as either high pass or low pass by a relay. In the following description, part references in parentheses refer to filter two. Relay K201, (K301) selects either the high- pass or low-pass configuration for all of the sixteen filters. The output of one R-C section is selected by multiplexer U202 or U203, (U301 or U302) and passed on to non-inverting buffer U202, (U303).
Approximately 80 pF input capacitance of the multiplexers is included in the calculation of the R-C time constants of the filters. The four highest frequency stages are not available as high-pass filters because of unacceptable attenuation of the signal that occurs when the filter capacitance forms a divider with the input capacitance of the multiplexers.
DG444 U205D, (U401A) is used to bypass the filter sections entirely and U101D, (U304D) is used to "reset" the filter stages by discharging them through R228, (R329).
U201, (U305) is the third, (fourth) gain stage with a fixed gain of 5. The input attenuator U205, (U304) allows setting the gain of these stages to 1, 2, or 5 under computer control.
OUTPUT STAGES
The fifth gain stage consists of op-amp U402 which is configured as a non-inverting amplifier with a gain of 5. U401 is a DG444 that again serves to switch the input attenuation of this stage for overall gains of 1, 2, or 5. Additionally, output offset
