ADR821/ADR827

Rev. 0 | Page 18 of 20

APPLICATIONS INFORMATION

+2.5 V AND −2.5 V OUTPUTS (ADR821)

REF

REF_OUT

AMP_OUT

V–

V+

10kΩ

10kΩ

+2.5V

–2.5V

+IN

0.1µF

Figure 38. +2.5 V and −2.5 V Outputs

In many dual-supply applications, it is desirable to have ±2.5 V

references. Using the configuration shown in Figure 38, it is

possible to generate −2.5 V with the help of a +2.5 V reference,

an internal op amp, and 10 kΩ resistors. The supply voltages V+

and V− should be greater than +2.8 V and −2.8 V, respectively.

The op amp is configured as an inverting amplifier with a gain

of −1, which produces −2.5 V at the output of the op amp. The

output of the reference is fed to the amplifier inverting input.

Because the op amp has very low input offset voltage (500 μV

over the full temperature range) and the TC ratio of the

resistors is typically ±25 ppm/°C, the −2.5 V output is less than

7 mV away from the theoretical value.

2.5 V AND 5.0 V OUTPUTS (ADR821)

REF_OUT

AMP_OUT

V–

V+

10kΩ

10kΩ

2.5V

5.0V

+IN

–IN

0.1µF

REF

Figure 39. 2.5 V and 5.0 V Outputs

In many single-supply applications, it is desirable to have

multiple reference voltages. Using the configuration shown in

Figure 39, it is possible to generate 5.0 V with the help of a

2.5 V reference, an internal op amp, and resistors. V+ should

be kept at greater than 5.8 V and V− can be connected either

to ground or to negative supply. The output of the reference is

fed to the amplifier noninverting input. The op amp is config-

ured as a noninverting amplifier with a gain of +2, which

produces 5 V at the output of the op amp. Using the guaranteed

maximum offset voltage over the temperature, and the typical

TC ratio of the resistors over the full temperature range, the

output is within 15 mV of the calculated value.

MULTIPLE 2.5 V OUTPUTS (ADR821)

REF_OUT

AMP_OUT

V–

V+

10kΩ

10kΩ

2.5V

2.5V

+IN

–IN

0.1µF

REF

Figure 40. Multiple 2.5 V Outputs

On some boards, sensitive analog circuits, such as a VCO, exist

with noisy digital circuits. If the supply current requirements

are low (less than 3 mA), series references and op amps can be

used. Using the configuration shown in Figure 40, two different

2.5 V supplies can be created using a single ADR821. The supply

voltage V+ should be greater than 2.8 V and V− can be con-

nected to ground or a negative voltage. The op amp is configured

as a voltage follower with a gain of +1, which produces 2.5 V at

the output of the op amp. The output of the reference is fed

to the amplifier noninverting input. Because the op amp has

very low input offset voltage (500 μV maximum over the full

temperature range), the output voltage from the op amp section

tracks the reference voltage within 1 mV. For a dynamic load,

such as the reference input pin on some analog-to-digital

converters, the load should be connected to an op amp output

and the noise sensitive circuitry, such as a VCO, should be

connected to the reference output. If the dynamic load is con-

nected to the reference voltage, any perturbations appear as a

signal to the input of the voltage follower and appear on the

other output.