Agilent 34970A/34972A Data Acquisition / Switch Unit

Chapter 5 Theory of Operation
Multifunction Module
The 74HC240’s, U205 and U206, provide the output high drive current
necessary to maintain a TTL high output level (= 2.4 VDC) under load.
At instrument turn-on, following a reset, and whenever the data lines
are being read, the MOSFETs are in the passive high state, and the high
output drivers are disabled. The resistor connected between the
MOSFET’s gate and ZGND holds the gate near ground potential when
the module is initially turned-on to ensure that the MOSFET is in the
passive high state.
The comparators U301, U302, U401 and U402 maintain correct TTL
high and low levels by shifting the voltages from the input to compensate
for the forward voltage drop of the blocking diode. A reference voltage of
+2.1 VDC (TTL_REF) is applied to the inverting input of the comparator.
When the input voltage is in the range of 0 VDC to +4.3 VDC the
blocking diode is forward biased and its forward voltage drop is added to
the applied voltage. For example, when 0 VDC is applied to the data line,
+0.7 VDC is present on the non-inverting input of the comparator and
the comparator output is low. When the input signal level is above 1.4
VDC, a voltage greater then +2.1 VDC is applied to the non-inverting
input of the comparator causing its output to go high. When the input
signal is less than 1.4 VDC, a voltage less than 2.1 VDC is applied to the
comparator’s non-inverting input causing its output to go low. This
ensures an input voltage < 1.4 VDC is interpreted as a TTL low level and
an input > 1.4 VDC is interpreted as a TTL high level.
The pull-up resistor (connected to the comparator’s non-inverting input)
allows external ground connections and open circuits to be detected.
When the data line is grounded, the blocking diode is forward biased
applying a +0.7 VDC level to the comparator, a TTL low. When the data
line is allowed to float, the non-inverting input of the comparator pulls
up to +5 VDC, a TTL high.
The blocking diode on the output is used for circuit protection. The diode
reverse biases when the applied voltage exceeds +4.3 VDC preventing
externally supplied current from being injected into the module’s +5V
supply line.
The MOSFETs have a built in zener diode that conducts at any voltage of
approximately 75 VDC or greater. The zener diode provides protection
from external over voltage situations including static electricity.
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