THEORY OF OPERATION
The following discussion refers to the schematic
below. A FET current source is used to bias a 6.3
zener diode. The zener voltage is divided by the
resistor network R1 and R2. This voltage is then
applied to the noninverting input of the operational
amplifier which amplifies the voltage to produce a
4.5000V output. The gain is determined by the
resistor networks R3 and R4: G=1 + R4/R3. The 6.3
zener diode is used because it is the most stable
diode over time and temperature.
The current source provides a closely regulated
zener current, which determines the slope of the
references’ voltage vs. temperature function.
By
trimming the zener current a lower drift over
temperature can be achieved. But since the voltage
vs.
temperature
function
is
nonlinear
this
compensation technique is not well suited for wide
temperature ranges.
Thaler Corporation has developed a nonlinear
compensation network of thermistors and resistors
that is used in the VRE series voltage references.
This proprietary network eliminates most of the
nonlinearity in the voltage vs. temperature function.
By then adjusting the slope, Thaler Corporation
produces
a
very
stable
voltage
over
wide
temperature ranges. This network is less than 2% of
the overall network resistance so it has a negligible
effect on long term stability. By using highly stable
resistors in our network, we produce a voltage
reference that also has very good long term
stability.
DISCUSSION OF PERFORMANCE
VRE204
FIGURE 1
APPLICATION INFORMATION
Figure 2 shows the proper connection of the
VRE204 series voltage references with the optional
trim resistors. Pay careful attention to the circuit
layout to avoid noise pickup and voltage drops in the
lines.
The VRE204 series voltage references have the
ground terminal brought out on two pins (pin 9 and
pin 10) which are connected together internally. This
allows the user to achieve greater accuracy when
using a socket. Voltage references have a voltage
drop across their power supply ground pin due to
quiescent
current
flowing
through
the
contact
resistance. If the contact resistance was constant
with time and temperature, this voltage drop could be
trimmed out. When the reference is plugged into a
socket, this source of error can be as high as 20ppm.
By connecting pin 10 to the power supply ground and
pin 9 to a high impedance ground point in the
measurement circuit, the error due to the contact
resistance can be eliminated. If the unit is soldered
into place, the contact resistance is sufficiently small
that it does not effect performance.
VRE204DS REV. C JUNE 1995
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