OP176 datasheet(13/21 Pages) AD

13 / 21 page

OP176

REV. 0

–13–

A Low Noise, +5 V/+10 V Reference

In many high resolution applications, voltage reference noise

can be a major contributor to overall system error. Monolithic

voltage references often exhibit too much wide band noise to be

used alone in these systems. Only through careful filtering and

buffering of these monolithic references can one realize wide-

band microvolt noise levels. The circuit illustrated in Figure 41

is an example of a low noise precision reference optimized for

both ac and dc performance around the OP176. With a +10 V

reference (the AD587), the circuit exhibits a 1 kHz spot output

noise spectral density < 10 nV/

√Hz. The reference output

voltage is selectable between 5 V and 10 V, depending only on

the selection of the monolithic reference. The output table

illustrated in the figure provides a selection of monolithic

references compatible with this circuit.

Figure 41. A Low Noise, +5 V/+10 V Reference

In operation, the basic reference voltage is set by U1, either a

5 V or 10 V 3-terminal reference chosen from the table. In this

case, the reference used is a 10 V buried Zener reference, but

all U1 IC types shown can plug into the pinout and can be

optionally trimmed. The stable 10 V from the reference is then

applied to the R1-C1-C2 noise filter, which uses electrolytic

capacitors for a low corner frequency. When electrolytic

capacitors are used for filtering, one must be cognizant of their

dc leakage current errors. Here, however, a dc bootstrap of C1

is used, so this capacitor sees only the small R2 dc drop as bias,

effectively lowering its leakage current to negligible levels. The

resulting low noise, dc-accurate output of the filter is then

buffered by a low noise, unity gain op amp using an OP176.

With the OP176’s low V

OS and control of the source resistances,

the dc performance of this circuit is quite good and will not

compromise voltage reference accuracy and/or drift. Also, the

OP176 has a typical current limit of 50 mA, so it can provide

higher output currents when compared to a typical IC reference

alone.

A Differential ADC Driver

High performance audio sigma-delta ADCs, such as the stereo

16-bit AD1878 and the 18-bit AD1879, present challenging

design problems with regards to input interfacing. Because of

an internal switched capacitor input circuit, the ADC input

structure presents a difficult dynamic load to the drive amplifier

with fast transient input currents due to their 3 MHz ADC

sampling rate. Also, these ADCs inputs are differential with a

rated full-scale range of

±6.3 V, or about 4.4 V rms. Hence, the

ADC interface circuit of Figure 42 is designed to accept a

balanced input signal to drive the low dynamic impedances seen

at the inputs of these ADCs. The circuit uses two OP176

Figure 42. A Balanced Driver Circuit for Sigma-Delta ADCs

amplifiers as inverting low-pass filters for their speed and high

output current drive. The outputs of the OP176s then drive the

differential ADC inputs through an RC network. This RC

network buffers the amplifiers against step changes at the ADC

sampling inputs using one differential (C3) and two common-

mode connected capacitors (C4 and C5). The 51

Ω series

resistors isolate the OP176s from the heavily capacitive loads,

while the capacitors absorb the transient currents. Operating on

±12 V supplies, this circuit exhibits a very low THD + N of

0.001% at 5 V rms outputs. For single-ended drive sources, a

third op amp unity gain inverter can be added between R2’s (+)

input terminal and R4. For best results, short-lead, noninduc-

tive capacitors are suggested for C3, C4, and C5 (which are

placed close to the ADC), and 1% metal-film types for R1

through R6. For surface mount PCBs, these components can

be NPO ceramic chip capacitors and thin-film chip resistors.

100pF

5.76k

Ω

5.62k

Ω

ΩR5

Ω

C2 100pF

5.49k

Ω

5.62k

Ω

ΩR6

Ω

0.01µF

0.0047µF

0.01µF

BALANCED

INPUTS

= AG, PIN 10 OR 18

VIN –

VIN +

AD1878/

AD1879

SIGMA-

DELTA

ADC

L & R

INPUTS

U1, U2 = OP176

–12V

ANALOG

(+)

(–)

NOTES

C1–C5 = NPO CERAMIC, NON-INDUCTIVE,

C3-C5 CLOSE TO ADC

R1–R6 = 1% METAL FILM

0.1µF

100µ/25V

+12V

ANALOG

COM

+VS

–VS

U1, U2

100µ/25V

(+)

USE

FOR

SINGLE-ENDED

INPUTS

Ω

OUTPUT TABLE

VOUT

10V

AD587

REF01

REF10

AD581

REF195

AD586

REF02

REF05

TOLERANCE

(+/–mV)

5 TO 10

30 TO 100

30 TO 50

5 TO 30

2 TO 10

2.5 TO 20

15 TO 50

15 TO 25

OP176

+15V

Ω

100

Ω

10k

Ω

0.1µF

100µF/25V

RTRIM

10k

Ω

100µF/25V

1.1k

Ω

100

Ω

3.3

Ω

10µF/25V

VOUT

REF

COMMON

(OPTIONAL)

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