DISCUSSION OF PERFORMANCE
THEORY OF OPERATION
The following discussion refers to the schematic
below. In operation, approximately 6.3 volts is
applied to the noninverting input of the op amp. The
voltage is amplified by the op amp to produce a
10.000V output. The gain is determined by the
networks R1 and R2: G=1 + R2/R1. The 6.3V zener
diode is used because it is the most stable diode
over time and temperature.
The zener operating current is derived from the
regulated output voltage through R3. This feedback
arrangement provides a closely regulated zener
current. This current determines the slope of the
reference's voltage vs. temperature function. By
trimming the zener current 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 VRE120 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.
APPLICATION INFORMATION
Figure 1 shows the proper connection of the
VRE120 series voltage reference with the optional
trim resistors. When trimming the VRE122, the
positive voltage should be trimmed first since the
negative voltage tracks the positive side. Pay careful
attention to the circuit layout to avoid noise pickup
and voltage drops in the lines.
When using the precision voltage references at
high temperatures it is best to keep them powered
up. If the zener diode isn't powered up at high
temperatures the junction will collect ions, and then
when power is applied, the voltage will drift until the
charge build up is depleted.
The VRE120 series voltage references have the
ground terminal brought out on two pins (pin 6 and
pin 7) 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 7 to the power supply ground and
pin 6 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 doesn't effect performance.
VRE120
FIGURE 2
VRE122
FIGURE 3
VRE120DS REV. C SEPT 1994
4-59
RHOPOINT [ RHOPOINT COMPONENTS ]
