Tektronix DM 5010, Инструкция по эксплуатации

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Theory of Operation— DM 5010
EXTERNAL
CONNECTORS
2994-10
Fig. 4-2. Simplified overvoltage protection and function switching.
DCV SIGNAL CONDITIONER <?>
The DCV Signal Conditioner provides range-dependent
gain for the various de mode-dependent signals applied to it.
Figure 4-3 shows a simplified diagram of the buffer amplifier
Table 4-1 shows the states of the buffer amplifier.
Table 4-1
DCV SIGNAL CONDITIONER GAIN SETTINGS
FET
Gain of the Buffer Amplifier
Switch
10
1
0.1
Q1105
ON OFF OFF
Q1106
OFF OFF
ON
Q1112 OFF ON ON
Q1114
ON ON OFF
gain of the buffer amplifier is set depending on which of the
FET switches are turned on by a high at their gates.
Feedback to the inverting input of the operational amplifier
is through either Q1112 or Q1105 while forward attenuation
of the signal to the A/D converter is determined by Q1114
for Q1106. Diodes CR1221 and CR1223 prevent
overdriving the buffer amplifier.
To ensure linear response of the buffer amplifier over its
input range, a bootstrap buffer is used to make the buffer
amplifier supply voltages track its input. This has the effect
of making U1210 operate at the middle of its range, avoiding
the linearity problems encountered when the output ap­
proaches one of the supply voltages. By making the gate
bias of the various FET switches also track the input volt­
age, proper bias is maintained and current leakage through
the FETs (and thus the associated error) is minimized. A
simplified schematic of the bootstrap buffer is shown in Fig.
4-4.
The Function Switch directs the mode-dependent de level
to the noninverting input of U1210, the buffer amplifier. The
The input to the bootstrap buffer, pin 3 of U1110, is con­
nected to the inverting input of U1210 and is therefore at the
4-6
ADD JAN 1982