VRE310 datasheet(30/41 Pages) ETC

30 / 41 page

DISCUSSION OF PERFORMANCE

VRE305DS REV. D MAY 2001

THEORY OF OPERATION

The following discussion refers to the schematic in

figure 2 below. A FET current source is used to bias a

6.3V zener diode. 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 5.000V output. The

gain is determined by the resistor networks R3 and R4:

G=1 + R4/R3. The 6.3V 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’

voltage 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

temperature ranges.

Thaler

Corporation

has

developed

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, Thaler Corporation produces a very stable voltage

over wide temperature ranges.

VRE305

FIGURE 2

This network is less than 2% of the overall network

resistance so it has a negligible effect on long term

stability.

Figure 3 shows the proper connection of the VRE305

series voltage references with the optional trim resistor for

initial error and the optional capacitor for noise reduction.

The VRE305 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 20ppm. 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.

EXTERNAL CONNECTIONS

FIGURE 3

+ V

OUT

+ V

VRE305

10KΩ

1µF

OPTIONAL

NOISE REDUCTION

CAPACITOR

OPTIONAL

FINE TRIM

ADJUSTMENT

V TEMP OUT

REF. GND

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