Theory of Operation
732B Circuit Description
4
4-7
The next major component that is used to generate 10V is the 10V resistor string. This
string (R401 and R402) is located on the Oven assembly. These are the feedback
elements that set the gain to multiply the reference voltage up to 10V. In its simplest
form, the voltage of the 10V output is expressed by the equation V
o
= (V
ref
)(1 +
R401/R402). The value of R401 is always 6.2 kΩ, and given the value of V
ref
, R402 is
wound to the value so that the 10V output equals 10V within ±1 mV.
But because of uncertainties and inaccuracies in these main components, the output
voltage can be as much as ±100 ppm (1 mV) from 10V. A high-resolution adjustment
circuit compensates for differences in components. Four bcd (binary-coded decimal)
switches allow the user to adjust the output in four decade ranges: 0.1, 1, 10, and 100
ppm.
The circuit controlled by the bcd switches is equivalent to an adjustable resistor in series
with the 10V feedback string resistor R402. As this adjustable resistor is varied, the
effective value of R402 varies, which varies the 10V output. This circuit allows the user
to adjust the output with 0.1 ppm resolution over a range of ±220 ppm. The components
that make up the 10V adjustment circuit are a thin-film resistor network (Z401 on the
Oven assembly), a 180Ω wirewound resistor (R405 on the Oven assembly), and resistors
R301 through R306 and SW301 through R304 on the A3 BCD Switch assembly.
Generating the 1
.
018V Output 4-7.
In addition to a 10V output, a 1.018V output is also provided. This output is derived by
resistively dividing down the 10V output. The 1.018V output is specified for close to 0
mA (20 pA) current load. The two resistors of this divider are R403 and R404 on the
Oven assembly. There is no adjustment of the 1.018V output, except by way of the 10V
output adjustment. The 1.108V output is expressed with the equation V
o
=
(10V)(R404)/(R403 + R404).
Biasing the Refamp for Low Temperature Coefficient 4-8.
As mentioned earlier, the Reference Amplifier contains an NPN transistor and a zener
diode in series. The TC (Temperature Coefficient) of the Reference Amplifier is the sum
of the TC of the zener voltage and the transistor base-emitter voltage. The zener voltage
TC is negative and the transistor TC is positive with a value dependent on its collector
current. Each Reference Amplifier is pretested to determine the collector current at
which the two TCs cancel out yielding an overall Reference Amplifier TC very close to
zero.
To generate this same collector current in the standard, a voltage of 2.976V is generated
across thin film resistor Z1-R3 on the Reference Hybrid (HR1). This resistor is
pretrimmed with a laser to the value that results in the correct collector current.
Sense Current Cancellation 4-9.
The purpose of the current cancellation circuits is to source current to or sink current
from components that are referenced to 10V sense low or connected to 10V sense high.
In the absence of the current cancellation circuits, bias current from these components
would flow through the 10V sense high and low lines to return to the power supply
common, degrading the performance of the 10V circuit.
Three different sense current cancellation circuits are used:
1. 10V sense high current cancellation circuit
2. 10V sense low current cancellation circuit
3. Reference Amplifier zener current cancellation circuit
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