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

VRE302

VRE302DS

67

3. tHEoRY of oPERAtion

The following discussion refers to the schematic in Figure 1. A FET current source is used to bias a 6.3 V zener di-

ode. 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 2.5 V output. The gain is determined by

the resistor networks R3 and R4: G=1 + R4/R3. The 6.3 V 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’ volt-

age 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 tem-

perature 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 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.

The proper connection of the VRE302 series voltage references with the optional trim resistor for initial error is

shown below. The VRE302 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 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 20 ppm. By con-

necting 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.

EXTERNal CONNECTIONS

PIN CONFIGuRaTION

8

4

6

5

2

VRE302

1µF

OPTIONAL

NOISE REDUCTION

CAPACITOR

OPTIONAL

FINE TRIM

ADJUSTMENT

3

7

REF. GND

5

6

7

8

VRE302

TOP

VIEW

1

2

3

4

N.C.

TEMP

GND

NOISE

REF. GND

TRIM