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

VRE405

4

VRE405DS

.ThEORYOFOPERaTION

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 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 VRE405 reference has it’s ground terminal brought out on two pins (pin 6 and 7) which are connected 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 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 resis-

tance can be eliminated. If the unit is soldered into place, the contact resistance is sufficiently small that it does not

effect performance.

PINCONFIGuRaTION

11

12

13

14

VRE405

TOP

VIEW

1

2

3

4

N/C

GND

REF. GND

5

6

7

10

9

8

OUT

N/C

IN

N/C

N/C

N/C

N/C

N/C

N/C