AD676

REV. A

–12–

AD586 output, thereby optimizing the overall performance of

the AD676. It is recommended that a 10

µF to 47 µF high qual-

AD676 and ground to minimize the impedance on the

reference.

+15V

+5V

–15V

100µF

100µF

100µF

AD676

10µF

10µF

0.1µF

0.1µF

0.1µF

78L12

79L12

0.01µF

0.01µF

NR

GND

10µF

0.1µF

1µF

AD587

10µF

15

17

18

12

16

4

2

6

8

10

Ω

10Ω

10

Ω

10Ω

Figure 7.

Using the AD676 with

±10 V input range (V

cally requires

±15 V supplies to drive op amps and the voltage

reference. If

±12 V is not available in the system, regulators

such as 78L12 and 79L12 can be used to provide power for the

AD676. This is also the recommended approach (for any input

range) when the ADC system is subjected to harsh environ-

ments such as where the power supplies are noisy and where

voltage spikes are present. Figure 7 shows an example of such a

system based upon the 10 V AD587 reference, which provides a

300

µV LSB. Circuitry for additional protection against power

supply disturbances has been shown. A 100

µF capacitor at each

regulator prevents very large voltage spikes from entering the

regulators. Any power line noise which the regulators cannot

eliminate will be further filtered by an RC filter (10

Ω/10 µF)

having a –3 dB point at 1.6 kHz. For best results the regulators

should be within a few centimeters of the AD676.

ANALOG INPUT

As previously discussed, the analog input voltage range for the

AD676 is

±V

(AGND SENSE) which allows a remote ground sense for the

input signal.

hibit dynamic characteristics. When a conversion cycle begins,

each analog input is connected to an internal, discharged 50 pF

capacitor which then charges to the voltage present at the corre-

sponding pin. The capacitor is disconnected when SAMPLE is

taken LOW, and the stored charge is used in the subsequent

conversion. In order to limit the demands placed on the external

source by this high initial charging current, an internal buffer

amplifier is employed between the input and this capacitance for

a few hundred nanoseconds. During this time the input pin ex-

hibits typically 20 k

Ω input resistance, 10 pF input capacitance

and

±40 µA bias current. Next, the input is switched directly to

the now precharged capacitor and allowed to fully settle. During

this time the input sees only a 50 pF capacitor. Once the sample

is taken, the input is internally floated so that the external input

source sees a very high input resistance and a parasitic input ca-

pacitance of typically only 2 pF. As a result, the only dominant

input characteristic which must be considered is the high cur-

rent steps which occur when the internal buffers are switched in

and out.

In most cases, these characteristics require the use of an external

op amp to drive the input of the AD676. Care should he taken

with op amp selection; even with modest loading conditions,

most available op amps do not meet the low distortion require-

ments necessary to match the performance capabilities of the

AD676. Figure 8 represents a circuit, based upon the AD845,

recommended for low noise, low distortion ac applications.

For applications optimized more for low bias and low offset than

speed or bandwidth, the AD845 of Figure 8 may be replaced by

the OP27.

499

Ω

1k

Ω

+12V

–12V

AD845

0.1

µF

0.1

µF

AGND

AGND

SENSE

±5V

INPUT

1k

Ω

AD676

15

13

14

2

3

4

7

6

V

IN

Figure 8.