AD676

REV. A

–11–

Using AGND SENSE to remotely sense the ground potential of

the signal source can be useful if the signal has to be carried

some distance to the A/D converter. Since all IC ground cur-

rents have to return to the power supply and no ground leads

are free from resistance and inductance, there are always some

voltage differences from one ground point in a system to

another.

Over distance this voltage difference can easily amount to sev-

eral LSBs (in a 10 V input span, 16-bit system each LSB is

about 0.15 mV). This would directly corrupt the A/D input sig-

nal if the A/D measures its input with respect to power ground

(AGND) as shown in Figure 5a. To solve this problem the

AD676 offers an AGND SENSE pin. Figure 5b shows how the

AGND SENSE can be used to eliminate the problem in Figure

5a. Figure 5b also shows how the signal wires should be

shielded in a noisy environment to avoid capacitive coupling. If

inductive (magnetic) coupling is expected to be dominant such

as where motors are present, twisted-pair wires should be used

instead.

The digital ground pin is the reference point for all of the digital

signals that operate the AD676. This pin should be connected

to the digital common point in the system. As Figure 4 illus-

trated, the analog and digital grounds should be connected to-

gether at one point in the system, preferably at the AD676.

V

IN

AGND

SOURCE

V

S

GROUND LEAD

TO POWER

SUPPLY GND

AD676

∆V

Figure 5a. Input to the A/D Is Corrupted by IR Drop in

V

IN

AGND

SENSE

AGND

SOURCE

SHIELDED CABLE

GROUND LEAD

TO POWER

SUPPLY GND

AD676

Figure 5b. AGND SENSE Eliminates the Problem in

Figure 5a.

VOLTAGE REFERENCE

The AD676 requires the use of an external voltage reference.

The input voltage range is determined by the value of the refer-

ence voltage; in general, a reference voltage of n volts allows an

input range of

±n volts. The AD676 is specified for both 10 V

and 5.0 V references. A 10 V reference will typically require

support circuitry operated from

±15 V supplies; a 5.0 V refer-

ence may be used with

±12 V supplies. Signal-to-noise perfor-

mance is increased proportionately with input signal range. In

the presence of a fixed amount of system noise, increasing the

LSB size (which results from increasing the reference voltage)

will increase the effective S/(N+D) performance. Figure 12

illustrates S/(N+D) as a function of reference voltage. In

contrast, INL will be optimal at lower reference voltage values

(such as 5 V) due to capacitor nonlinearity at higher voltage

values.

During a conversion, the switched capacitor array of the AD676

presents a dynamically changing current load at the voltage ref-

erence as the successive-approximation algorithm cycles through

various choices of capacitor weighting. (See the following sec-

characteristics.) The output impedance of the reference circuitry

must be low so that the output voltage will remain sufficiently

constant as the current drive changes. In some applications, this

may require that the output of the voltage reference be buffered

by an amplifier with low impedance at relatively high frequen-

cies. In choosing a voltage reference, consideration should be

made for selecting one with low noise. A capacitor connected

between REF IN and AGND will reduce the demands on the

reference by decreasing the magnitude of high frequency com-

ponents required to be sourced by the reference.

Figures 6 and 7 represent typical design approaches.

10

µF

AGND

1.0

µF

+12V

AD586

AD676

13

16

6

2

4

8

+

Figure 6.

Figure 6 shows a voltage reference circuit featuring the 5 V out-

put AD586. The AD586 is a low cost reference which utilizes a

buried Zener architecture to provide low noise and drift. Over

the 0

°C to +70°C range, the AD586L grade exhibits less than

2.25 mV output change from its initial value at +25

°C. A noise-