AD7874SE View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD7874SE

LC2MOS 4-Channel, 12-Bit Simultaneous Sampling Data Acquisition System

Analog Devices 

AD7874

The first read operation to the AD7874 after conversion always

accesses data from Data Register 1 (i.e., the conversion result

from the VIN1 input). INT is reset high on the falling edge of

RD during this first read operation. The second read always ac-

cesses data from Data Register 2 and so on. The address pointer

is reset to point to Data Register 1 on the rising edge of

CONVST. A read operation to the AD7874 should not be at-

tempted during conversion. The timing diagram for the

AD7874 conversion sequence is shown in Figure 7.

CONVST

INT

CS

RD

DATA

TRACK/HOLDS GO

INTO HOLD

t1

tCONV

tACQUISITION

t2

t6

HIGH-IMPEDANCE

t5

t8

t4

t3

t7

CH1 HIGH- CH2 HIGH- CH3 HIGH- CH4

DATA Z DATA Z DATA Z DATA

HIGH-Z

TIMES t2, t3, t4, t6, t7, AND t8 ARE THE SAME FOR ALL FOUR READ OPERATIONS.

Figure 7. AD7874 Timing Diagram

AD7874 DYNAMIC SPECIFICATIONS

The AD7874 is specified and 100% tested for dynamic perfor-

mance specifications as well as traditional dc specifications such

as Integral and Differential Nonlinearity. These ac specifications

are required for the signal processing applications such as

phased array sonar, adaptive filters and spectrum analysis.

These applications require information on the ADC’s effect on

the spectral content of the input signal. Hence, the parameters

for which the AD7874 is specified include SNR, harmonic dis-

tortion, intermodulation distortion and peak harmonics. These

terms are discussed in more detail in the following sections.

Signal-to-Noise Ratio (SNR)

SNR is the measured signal to noise ratio at the output of the

ADC. The signal is the rms magnitude of the fundamental.

Noise is the rms sum of all the nonfundamental signals up to

half the sampling frequency (fs/2) excluding dc. SNR is depen-

dent upon the number of quantization levels used in the digiti-

zation process; the more levels, the smaller the quantization

noise. The theoretical signal to noise ratio for a sine wave input

is given by

SNR = (6.02N + 1.76) dB

(1)

where N is the number of bits.

Thus for an ideal 12-bit converter, SNR = 74 dB.

The output spectrum from the ADC is evaluated by applying a

sine wave signal of very low distortion to the VIN input which is

sampled at a 29 kHz sampling rate. A Fast Fourier Transform

(FFT) plot is generated from which the SNR data can be ob-

tained. Figure 8 shows a typical 2048 point FFT plot of the

AD7874BN with an input signal of 10 kHz and a sampling

frequency of 29 kHz. The SNR obtained from this graph is

73.2 dB. It should be noted that the harmonics are taken into

account when calculating the SNR.

Figure 8. AD7874 FFT Plot

Effective Number of Bits

The formula given in Equation 1 relates the SNR to the number

of bits. Rewriting the formula, as in Equation 2, it is possible to

get a measure of performance expressed in effective number of

bits (N).

N = SNR − 1.76

6.02

(2)

The effective number of bits for a device can be calculated di-

rectly from its measured SNR.

Figure 9 shows a typical plot of effective number of bits versus

frequency for an AD7874BN with a sampling frequency of

29 kHz. The effective number of bits typically falls between

11.75 and 11.87 corresponding to SNR figures of 72.5 dB and

73.2 dB.

z

Figure 9. Effective Numbers of Bits vs. Frequency

–8–

REV. C

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