Tektronix Option 10 Service Manual page 162

DMM Option-Theory of Operation
24x58124678 Options Service
DC VOLTS BUFFER. The DC Volts Buffer buffers dc
input voltages, sending the resultant signal to the V/F
lnput Multiplexer.
lnput voltages first pass by U5060B, an active low pass
filter. It removes both input noise and FET switching noise
from the input signal. FET Switch Q5070A and Q5070B,
selects either the unknown input voltage or the voltage
reference (OFFSET or INPUT REF). Operational amplifier
U5060A maintains proper bias on the FET switch, with
varying input voltages. The 85 and Digital Control sig-
nals control the FET switch.
Operational amplifier U4970 amplifies the selected input
signal. FET switches U4950C and U4950D control the
feedback resistance and therefore the gain of the opera-
tional amplifier. The B6 Digital Control signal controls the
FET switches. A LO on the control input (pin 16 or pin 9)
of one of the FET switches closes the switch.
AC VOLTS BUFFER. The AC Volts Buffer buffers ac
input voltages, converts the ac voltage to dc, and then
sends the resultant signal to the V/F lnput Multiplexer.
Operational amplifier U5151 B buffers the ac input volt-
age. VR5160, VR5162, R5167, R5168, CR5163, and
CR5164 protect the amplifier's input. The output of the
operational amplifier is sent to operational amplifier
U5151A. FET switches U5150C and U5150D control the
operational amplifier's feedback resistance and therefore
its gain. The C7 Digital Control signal controls the FET
switches. A LO on the control input (pin 16 or pin 9) of one
of the FET switches closes the switch. The output of the
operational amplifier is converted to dc ,by rms-to-dc con-
verter U5140.
V/F INPUT MULTIPLEXER. The V/F lnput Multiplexer
selects one signal from the DMM lnput Circuit. The
selected signal is sent to the V/F Converter. Signal
selection is controlled by Digital Control signals 82, B3,
and B4. The signal selected is either the output of the AC
Volts Buffer (AC XI or AC +lo), the output of the DC
Volts Buffer (DC XI or DC
+
1 O), the -2 V REF, the
Ground REF, or the AMPS ST signal.
OHMS CURRENT SOURCE. The Ohms Current Source
generates the constant currents used to make resistance
measurements. Also contained in the circuitry are the
voltage references used by the current source and those
used in all measurement sequences.
The voltage references are produced by U5050, R5049,
R5054, R5055, and R5056. The Ohms Current Source
uses the
-
6.95 V reference. The -2.0 V or -0.20 V
reference is measured during reference measurement
cycles. FET switch U4942B selects one of the references.
The A6 Digital Control signal controls the FET switch. FET
switch U4942A selects either the selected reference or the
ground offset. The offset is measured during an offset
measurement cycle. The A5 Digital Control signal controls
the FET switch. For ac measurements, the -2.0 V refer-
ence is always used, and the V/F lnput Multiplexer selects
the -2.0 V reference and the offset directly.
The voltage drop across R4951 determines the current
through Q4952. The voltage reference of -6.95 V is at
one end of the resistor. FET switch U4942C, controlled by
the A7 Digital Control signal, selects either -6.26 V or
0.0 V for the other end of the resistor. Voltage follower
U5040 buffers the selected voltage.
The resulting current through (24952 (either 1 mA or
0.1 mA) is divided by either 1 or 10 by R4957 and FET
switches U4950A and U4950B, the negative feedback loop
for operational amplifier U4960. The positive feedback loop
for U4960 drops the same voltage as its negative feed-
back loop. The B O Digital Control signal controls the nega-
tive feedback; the B1 Digital Control signal controls the
positive feedback. The selections for each Ohms range are
shown in Table 4-4.
The Voltage Clamp, CR4980 and CR4981, keeps the
output voltage between -0.7 V and 5.7 V and protects
the current source from over-voltage inputs.
V/F Converter and Digital Control
The V/F Converter and Digital Control circuitry (see
Diagram 29) generates a frequency that is inversely pro-
portional to the voltage received from the input circuit. It
also contains the registers which control the DMM lnput
Circuit hardware.
VOLTAGE-TO-CURRENT CONVERTER. The V/F lnput
Multiplexer (U5020, Diagram 28) selects the input to the
Voltage-to-Current Converter. Selected input is converted
to a current and inverted by operational amplifiers U5030A
and U5030B. The current, which is inversely proportional
to the input voltage, passes through Q4934 and charges
integrating capacitor C4914 negatively.
INTEGRATING CAPACITOR. Integrating Capacitor
C4914 is charged negatively by the Voltage-to-Current