VRE304 데이터 시트보기 (PDF) - Unspecified

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VRE304

Low Cost Precision Reference

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DISCUSSION OF PERFORMANCE

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 4.500V 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 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, Thaler Corporation produces a very stable voltage

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.

Figure 3 shows the proper connection of the VRE304

series voltage references with the optional trim resistor

for initial error and the optional capacitor for noise

reduction. The VRE304 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.

VRE304

FIGURE 2

FIGURE 3

EXTERNAL CONNECTIONS

OPTIONAL

NOISE REDUCTION

CN

1µF

CAPACITOR

+ VIN

V TEMP OUT

2

3

8

6

VRE304

5

7

4

+ VOUT

10kΩ

OPTIONAL

FINE TRIM

ADJUSTMENT

REF. GND

VRE304DS REV. C AUG. 1995

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