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

VRE202

4

VRE202DS

3. tHEoRY of oPERAtion

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

diode. The zener voltage is divided by the resistor network R1 and R2. This voltage is then applied to the noninvert-

ing 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. Then by adjusting

the slope, a very stable voltage over wide temperature ranges is produced. 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, a voltage reference is produced that also has very good long term stability.

4. aPPlICaTION INFORMaTION

The proper connection of the VRE202 series voltage references with the optional trim resistors is shown below. Pay

careful attention to the circuit layout to avoid noise pickup and voltage drops in the lines.

The VRE202 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 20 ppm. By con-

necting 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.

EXTERNal CONNECTIONS

PIN CONFIGuRaTION

11 12 13

9 10

18

17

16

15

14

4

5

6

7

8

1 20 19

3

2

+15V

10KΩ

Ref. Gnd.

-15V

TOP VIEW

VRE202

4

5

6

7

8

19

20

1

2

3

NC

NC

TRIM

GND

REF GND

NC V

IN

NC NC NC

NC

NC

NC

NC

NC

13

12

11

10

9

NC NC NC V

18 17 16 15 14

Δ