Agilent Technologies 2712A, 2948 manual Simplified Diagram of the Sampler Amplifier

Service Model 8904A

+15V	+15v
& R213	I
c201	+ TO	LOW-PASS
II
11		FILTER

rDAC-ANLSA’LIPLER - 1

R216

I;+(f\;I iz

L - - - - I

t T H l S LINE IS AT GUARD TRACE POTENTIAL

Figure 8-10.Simplified Diagram of the Sampler Amplifier

Sampler Amplifier

The Sampler Amplifier must accurately track the DAC output voltage when the Sampler Switch is closed and faithfully hold the sampled voltage when the switch is open. For some waveforms (for example, square and ramp) the DAC output may change abruptly several volts; these transitions must pass through the Sampler Amplifier without distortion.

Refer to the simplified schematic of Figure 8-10which shows the Sampler Amplifier without the guard traces. The capacitor which holds the sampled voltage from the DAC is C219. It is followed by a unity-gain amplifier which presents a very ‘high impedance to C219 (so as not to discharge it during the “hold” period), responds quickly and stably to a wide variety of inputs, has no dc offset, and can drive the load at its output.

The amplifier can be divided into two main stages: (1) a slower, high-gain input stage (U203 and associated parts) and (2) a fast, lower-gain output stage (Q210 and following). The input stage assures sufficient gain accuracy; the output stage assures quick response and output drive.

For high-frequency signal components, capacitor C201 bypasses the input stage so that the input signal appears at the gate of FET Q210. A high input impedance is maintained because capacitors C226 and C225 short resistors R216 and R218 respectively to the output of the output stage which is a unity-gain follower. Both ends of R216 and R218 move in step with the input so no current flows through them. Capacitor C217 prevents high-frequency components from entering U203. Capacitor C224 reduces the high-frequency stage gain to unity to eliminate its influence.

For low-frequency components, the input signal appears at the non-inverting (+) input of U203. The high, open-loop gain of U203 and the unity feedback force the inverting (->input of U203 to precisely follow the non-inverting input.

Refer now to Service Sheet 1. The guard traces associated with U203 prevent any significant leakage of current from nearby circuit nodes of different potential on to highly-sensitive nodes such as C219. Leakage is most severe when the humidity is high. The guard trace is at the same potential as the input signal at all times since it is connected to the output of U202. Potentially error-producing leakage currents flow harmlessly to the guard trace.

8-26 Principles of Operation (A3)