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ADR525/ADR530/ADR550

Rev. F | Page 9 of 12

THEORY OF OPERATION

The ADR525/ADR530/ADR550 use the band gap concept to

produce a stable, low temperature coefficient voltage reference

suitable for high accuracy data acquisition components and

systems. The devices use the physical nature of a silicon transistor

base-emitter voltage (VBE) in the forward-biased operating region.

All such transistors have approximately a −2 mV/°C tempera-

ture coefficient (TC), making them unsuitable for direct use as

low temperature coefficient references. Extrapolation of the

temperature characteristics of any one of these devices to

absolute zero (with the collector current proportional to the

absolute temperature), however, reveals that its VBE approaches

approximately the silicon band gap voltage. Thus, if a voltage

develops with an opposing temperature coefficient to sum the

VBE, a zero temperature coefficient reference results. The

ADR525/ADR530/ADR550 circuit shown in Figure 14 provides

such a compensating voltage (V1) by driving two transistors at

different current densities and amplifying the resultant VBE

difference (ΔVBE, which has a positive temperature coefficient).

The sum of VBE and V1 provides a stable voltage reference over

temperature.

VBE

–

∆VBE

–

V–

V+

–

Figure 14. Circuit Schematic

APPLICATIONS

The ADR525/ADR530/ADR550 are a series of precision shunt

voltage references. They are designed to operate without an

external capacitor between the positive and negative terminals.

If a bypass capacitor is used to filter the supply, the references

remain stable.

All shunt voltage references require an external bias resistor (RBIAS)

between the supply voltage and the reference (see Figure 15).

RBIAS sets the current that flows through the load (IL) and the

reference (IIN). Because the load and the supply voltage can vary,

RBIAS needs to be chosen based on the following considerations:

•

RBIAS must be small enough to supply the minimum IIN

current to the ADR525/ADR530/ADR550, even when the

supply voltage is at its minimum value and the load current

is at its maximum value.

•

RBIAS must be large enough so that IIN does not exceed

15 mA when the supply voltage is at its maximum value

and the load current is at its minimum value.

•

VOUT

ADR550

IIN

IIN + IL

Figure 15. Shunt Reference

Given these conditions, RBIAS is determined by the supply

voltage (VS), the load and operating currents (IL and IIN) of

the ADR525/ADR530/ADR550, and the output voltage (VOUT)

of the ADR525/ADR530/ADR550.

OUT

BIAS

−

(3)

Precision Negative Voltage Reference

The ADR525/ADR530/ADR550 are suitable for applications

where a precise negative voltage is desired. Figure 16 shows the

ADR525 configured to provide a negative output.

–2.5V

ADR525

Figure 16. Negative Precision Reference Configuration

Output Voltage Trim

The trim terminal of the ADR525/ADR530/ADR550 can be

used to adjust the output voltage over a range of ±0.5%. This

allows systems designers to trim small system errors by setting

the reference to a voltage other than the preset output voltage.

An external mechanical or electrical potentiometer can be used

for this adjustment. Figure 17 illustrates how the output voltage

can be trimmed using the AD5273, an Analog Devices, Inc.,

10 kΩ potentiometer.

470kΩ

POTENTIOMETER

10kΩ

ADR530

AD5273

VOUT

Figure 17. Output Voltage Trim

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