Tektronix DM 5010, Инструкция по эксплуатации

Страница: 83 / 347

Theory of Operation—DM 5010
input
BUFFER
2994-13
Fig. 4-5. Simplified charge-balancing converter.
Figure 4-7 illustrates the currents at the summing node
when the zero-reference is being set. The node current from
the Converter Input buffer, U1 120, may be ignored since it is
very small during the Auto-Zero period.
As the Auto-Zero period begins, a counter in the Timing
Logic is set to zero. The Reference Buffer input is then
toggled by TO and T8 with a 50% duty cycle between
and ground, injecting an average current l^ in equal to
+ V<ef/ (2 X R1231) into the summing node. The Integrator
produces a voltage output to the Auto-Zero Buffer. The
Auto-Zero Buffer sinks current, via R1233, of opposite po­
larity to the current injected by the Reference Buffer until the
current removed from the node balances the current being
injected. This is the equilibrium state, and the voltage
waveform at the input of the Auto-Zero Buffer is that re­
quired to precisely track the injected current input. Resistors
R1139, R1227, and capacitor C1139 at the Auto-Zero Buff­
ers input integrate the integrator output waveform over a
preset number of measure intervals. The negative Auto­
Zero Buffer output voltage across R1233 generates a cur­
rent equal in magnitude but of opposite polarity to the
average injected reference current. The averaged voltage at
the input of the Auto-Zero Buffer is the zero-reference volt-
ADD JAN 1982
4-9