AD7896

REV. B

–11–

Dynamic Performance (Mode 1 Only)

With a combined conversion and acquisition time of 9.5

µs, the

AD7896 is ideal for wide bandwidth signal processing applica-

tions. These applications require information on the ADC’s ef-

fect on the spectral content of the input signal. Signal to (Noise

+ Distortion), Total Harmonic Distortion, Peak Harmonic or

Spurious Noise and Intermodulation Distortion are all specified.

Figure 11 shows a typical FFT plot of a 10 kHz, 0 V to +3.3 V

input after being digitized by the AD7896 operating at a

102.4 kHz sampling rate. The Signal to (Noise + Distortion)

Ratio is 71.5 dB and the Total Harmonic Distortion is –83 dB.

–0

–120

51200

–60

–100

10240

–80

0

–20

–40

40960

30720

20480

F

F

SNR = 71.54dB

THD = –82.43dB

FREQUENCY – Hz

Figure 11. AD7896 FFT Plot

Effective Number of Bits

The formula for Signal to (Noise + Distortion) Ratio (See Ter-

minology section) is related to the resolution or number of bits

in the converter. Rewriting the formula, below, gives a measure

of performance expressed in effective number of bits (N):

N = (SNR 1.76)/6.02

where SNR is Signal to (Noise + Distortion) Ratio.

The effective number of bits for a device can be calculated from

its measured Signal to (Noise + Distortion) Ratio. Figure 12

shows a typical plot of effective number of bits versus frequency

quency is 102.4 kHz. The plot shows that the AD7896 converts

an input sine wave of 51.2 kHz to an effective numbers of bits of

11.25 which equates to a Signal to (Noise + Distortion) level

of 69 dB.

12.00

0

51.2

25.6

11.75

11.50

11.25

11.00

INPUT FREQUENCY – kHz

Figure 12. Effective Number of Bits vs. Frequency

Power Considerations

In the automatic power-down mode then the part may be oper-

ated at a sample rate which is considerably less than 100 kHz. In

this case the power consumption will be reduced and will de-

pend on the sample rate. Figure 13 shows a graph of the power

consumption versus sampling rates from 10 Hz to 1 kHz in the

automatic power-down mode. The conditions are 2.7 V supply

25

°C, serial clock frequency of 8.33 MHz and the data was read

after conversion.

200

0

1000

120

40

100

80

10

160

SAMPLING RATE IN Hz

F

Figure 13. Power vs. Sample Rate in Auto Power-Down

Mode