REV. A

AD974

–17–

TEMPERATURE – C

110

80

–75

150

–50

–25

0

25

50

75

100

125

105

100

95

90

85

–80

–110

–85

–90

–95

–100

–105

SNRD

SFDR

THD

Figure 20. AC Parameters vs. Temperature

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 AD974 input, and

10,000 conversions are recorded, the result will be a distribution

of codes as shown in Figure 21. This histogram shows a bell

shaped curve consistent with the Gaussian nature of thermal

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

0

–3

–2

–1

0

12

34

3500

2000

1500

1000

500

3000

2500

Figure 21. Histogram of 10,000 Conversions of a DC Input

POWER-DOWN FEATURE

The AD974 has analog and reference power-down capability

through the PWRD pin. When the PWRD pin is taken high,

the power consumption drops from a maximum value of

100 mW to a typical value of 50

µW. When in the power-

down mode the previous conversion results are still available in

the internal registers and can be read out providing it has not

already been shifted out.

When used with an external reference, connected to the REF

pin and a 2.2

µF capacitor, connected to the CAP pin, the

power-up recovery time is typically 1 ms. This typical value of

1 ms for recovery time depends on how much charge has de-

cayed from the external 2.2

µF capacitor on the CAP pin and

assumes that it has decayed to zero. The 1 ms recovery time has

been specified such that settling to 16 bits has been achieved.

When used with the internal reference, the dominant time con-

stant for power-up recovery is determined by the external ca-

pacitor on the REF pin and the internal 4K impedance seen at

that pin. An external 2.2

µF capacitor is recommended for the

REF pin.

CROSSTALK

The crosstalk between adjacent channels, nonadjacent channels

and worst-case adjacent channels is shown in Figures 22 to 24.

The worst-case crosstalk occurs between channels 1 and 2.

–80

–115

1

10

100

1000

–95

–100

–105

–110

–85

–90

10000

ACTIVE CHANNEL INPUT FREQUENCY – kHz

ADJACENT CHANNELS,

WORST PAIR

NONADJACENT

CHANNELS

Figure 22. Crosstalk vs. Input Frequency (kHz)

0

–130

12

–90

–100

–110

–120

–70

–80

FREQUENCY – kHz

4

6

8

10

12

14

16

18

20

–60

–40

–50

–30

–10

–20

Figure 23. Adjacent Channel Crosstalk, Worst Pair

(8192 Point FFT; AIN 2 = 1.02 kHz, –0.1 dB; AIN 1 = AGND)