AD976/AD976A

–13–

REV. C

DC CODE UNCERTAINTY

Ideally, a fixed dc input should result in the same output code

for repetitive conversions; however, as a consequence of un-

avoidable circuit noise within the wideband circuits of the ADC,

a range of output codes may occur for a given input voltage.

Thus, when a dc signal is applied to the AD976/AD976A input,

and 10,000 conversions are recorded, the result will be a distri-

bution of codes as shown in Figure 19. This histogram shows a

bell-shaped curve consistent with the Gaussian nature of ther-

mal noise. The histogram is approximately seven codes wide.

The standard deviation of this Gaussian distribution results in a

code transition noise of 1 LSB rms.

4000

3500

0

–3

2000

1500

1000

500

3000

2500

–2

–1

0

1

2

3

4

Figure 19. Histogram of 10,000 Conversions of a DC Input

MICROPROCESSOR INTERFACING

The AD976/AD976A is ideally suited for traditional dc mea-

surement applications supporting a microprocessor and ac signal

processing applications interfacing to a digital signal processor.

The AD976/AD976A is designed to interface with a 16-bit data

bus and provides all output data bits in a single read cycle. A

variety of external buffers can be used with the AD976/AD976A

to prevent bus noise from coupling into the ADC. The following

sections illustrate the use of the AD976/AD976A with the

MC68000 and 8051 microcontrollers and the TMS320C25 and

ADSP-2111 signal processors.

MC68000 Interface

Figure 20 shows a general interface diagram for the MC68000

16-bit microprocessor to the AD976/AD976A. In Figure 20,

conversion is initiated by bringing CSA low (i.e., writing to the

appropriate address). This allows the processor to maintain

control over the complete conversion process.

ADDRESS BUS

*ADDITIONAL PINS OMITTED FOR CLARITY

DATA BUS

DB15

DB0

BUSY

AD976/

AD976A

R/

C

BUS

74HC374

OE

Q15

Q0

D0

D15

CLK

EN

ADDR

DECODE

A0

A15

D15

D0

68000

R/W

AS

Figure 20. AD976/AD976A to 68000 Interface

250

0

150

100

200

50

POSITIVE DNL DISTRIBUTION – LSB

Figure 17. Typical Position DNL Distribution (1516 Units)

140

0

80

20

120

100

NEGATIVE DNL DISTRIBUTION – LSB

60

40

Figure 18. Typical Negative DNL Distribution (1516 Units)