Acconverter; Scaling; Acconversion; Ohmsconversion - Keithley 177 Instruction Manual

2. In the ACV mode, AC inputs for all ranges pass through
the AC converter which consists of an amplifier/attenuator
and a TRMS converter, as shown in Figure 4-2. The AC in-
put is scaled by the amplifier/attenuator to 2V rms full
scale for all ranges. The TRMS converter provides a
positive DC output signal which is proportional to the true
root mean square AC signal. This DC signal is the input to
the A/D converter. In the ACA mode, the shunt voltage is
applied to the input of the AC converter. The amplifier/at-
tenuator provides a x 10 gain for all AC current ranges.
3. Ohms conversion-Resistance measurements ara made by
supplying a constant current from the ohms converter to
the unknown resistor (Rx). The resulting voltage developed
across the unknown resistor is proportional to its
resistance, and this voltage is applied to the input of the
chopper amplifier used for DC volts. The chopper amplifier
output to the A/D converter is scaled to -2V full scale for
all ohms ranges. Scaling of the ohms signal over the seven
ranges is accomplished by using five constant current
levels and the three gains of the chopper amplifier.
4.2.2 AID Converter
The A/D converter is a large scale integration (LSI)
ratiometric device. Converter output is a multiplexed five digit
number, in binary coded decimal IBCD) form which is equal
to the ratio of the input voltage to the reference voltage. A
separate clock circuit supplies a 1OOkHz timing input to the in-
tegrated circuit, which also multiplexes the BCD output. The
full scale AID input for all ranges of the Model 177 is 2VDC.
with the polarity determined by the function as follows: ACV
and ACA. +2V; DCV and DCA. rt2V; Resistance (0). -2V.
4.2.3 Range Switching
It is important to note that the range switches on schematic
29658E are identified by the front panel marking for the
L2 ranges. The corresponding V and A ranges may either be
determined from the instrument front panel or by using Table
4-l.
4.3 AC CONVERTER
When making AC voltage measurements, the AC converter
scales the input signal by appropriate attenuation or
amplification, performs TRMS conversion and provides the
equivalent positive DC level to the A/D for display. Input
resistance for AC volts is 1MD IR107). Shunt capacitance is
less than 75pF. Signal conditioning is as follows:
4.3.1 Scaling
Scaling
is performed in the first two stages, as shown on
schematic drawing number 29658E and summarized in Table
42. Scaling is performed by using a relay and FET switches
to switch feedback resistors in the attenuator IUlOl) and the
amplifier (U102). The actual gains used in attenuator UlOl
are 0.99, 0.0099 and 0.00099. Amplifier U102 gains are
nominally 1.003 and 10.03. The combined gain of UlOl and
U102 is offset low to permit gain adjustment in the TRMS
stage. The only adjustments provided in the attenuator and
amplifier stages are two high frequency compensation
capacitors (Cl02 and C103). AC coupling is used at the input
and output of the attenuator and amplifier stages. Input
overload protection is provided by diodes (CRlOGand CR1071
and resistors R107 (1MQ. 2WI.
4.3.2
AC Conversion
AC-to-DC conversion is performed b a monolithic TRMS
module lU103). Output V,
output filtering are done be ore the internal output buffer to
F =&. Averaging and
give a low impedance for the analog output. Potentiometer
R113 provides midband (10K count) gain adjustment and
R114 establishes output zero (1K count). Capacitor Cl08 is
used in the averaging circuit, and Cl09 provides filtering as
described above.
4.3.3 AC Current Measurements
For AC current measurements, the appropriate shunt resistor
is placed in front of the AC converter. The 200mV range gain
configuration is used for all current ranges.
4.4 DC SIGNAL
CONDITIONING
For DC voltage measurements, the input resistance is lOMO,
which is established by all of the range resistors being con-
nected in series to signal ground. On low voltage ranges, the
DC input is applied to the top of the resistive divider (range
resistor) and to the buffer amplifier input without attenuation.
Also, the chopper amplifier has appropriate gain to provide a
f2V full scale output to the A/D. On high voltage ranges,
the DC signals are scaled down to *2V full scale by the
resistive divider and the chopper is used in the unity-gain
Ifollower) mode as a buffer. A summary of input attenuation
and chopper amplifier gain is given in Table 4-3.
Table 4-l. Range Switch Correlation
Function Ranges
0
20 200 2k 20k 200k 2000k 20M
V 20mX 200m 2 20 200 1000**
A 20/L* 2oofl
2m 20m 200m 2000m
*DC Only
l
*ACV 1000, DCV 1200
4-2