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

VRE3025

4

VRE3025DS

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 that 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 VRE3025 series voltage references with the optional trim resistor for initial error and the

optional capacitor for noise reduction is shown below.

OUTPUT NOISE-VOLTAGE

DENSITY vs. FREQUENCY

CHANGE IN OUTPUT VOLTAGE

VS. INPUT VOLTAGE

CHANGE IN OUTPUT VOLTAGE

VS. OUTPUT CURRENT

Frequency (Hz)

12 13

9

15

20

0

-20

14

16

11

10

0

-10

10

30

40

50

60

8 10

2

14

0

-200

-400

12

16

6

4

0

-300

-100

100

200

300

400

1k

10k

40

30

20

10

50

100

10

0.1Hz to 10Hz Noise

1 Sec/Div