P r o d u c t I n n o v a t i o n F r o m

VRE310



VRE310DS

3.ThEORYOFOPERaTION

The following discussion refers to the block diagram in Figure 1. In operation, approximately 6.3 V is applied to

the noninverting input of the op amp. The voltage is amplified by the op amp to produce a 10 V output. The gain is

determined by the networks R1 and R2: G=1 + R2/R1. The 6.3 V 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 references’ voltage vs. tempera-

ture 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.

A nonlinear compensation network of thermistors and resistors 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, a very stable voltage is produced 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.

The proper connection of the VRE310 series voltage references with the optional trim resistor is shown below. The

VRE310 reference has the ground terminal brought out on two pins (pin 4 and pin 7) which are connected together

internally. This allows the user to achieve greater accuracy when using a socket. Voltage references have a volt-

age 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 20 ppm. By connecting pin 4 to the power

supply ground and pin 7 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. Pay careful attention to the circuit layout to avoid noise pickup and voltage drops in the

lines.

EXTERNalCONNECTIONS

PINCONFIGuRaTION

8

4

6

5

2

VRE310

1µF

OPTIONAL

NOISE REDUCTION

CAPACITOR

OPTIONAL

FINE TRIM

ADJUSTMENT

3

7

REF. GND

5

6

7

8

VRE310

TOP

VIEW

1

2

3

4

N/C

TEMP

GND

NOISE

REDUCTION

REF. GND

TRIM