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AD7870AJNZ

LC2MOS Complete, 12-Bit, 100 kHz , Sampling ADC

Analog Devices 

AD7870A

Figure 10. Mode 1 Timing Diagram, Byte or Serial Read

The timing diagram for byte and serial data is shown in Figure

10. INT goes low at the end of conversion and is reset high by

the first falling edge of CS and RD. This first read at the end of

conversion can either access the low byte or high byte of data

depending on the status of HBEN (Figure 10 shows low byte

only for example). The diagram shows both a noncontinuously

and a continuously running clock (dashed line).

AD7870A DYNAMIC SPECIFICATIONS

The AD7870A is specified and 100% tested for dynamic perfor-

mance specifications. These ac specifications are required for

signal processing applications such as speech recognition, spec-

trum analysis and high speed modems. These applications re-

quire information on the ADC’s effect on the spectral content

of the input signal. Hence, the parameters for which the AD7870A

is specified include SNR, harmonic distortion, 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 con-

verter, 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 100 kHz sampling rate. A Fast Fourier Transform

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

tained. Figure 11 shows a typical 2048 point FFT plot of the

AD7870AJN with an input signal of 25 kHz and a sampling

frequency of 100 kHz. The SNR obtained from this graph is

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

account when calculating the SNR.

Figure 11. FFT Plot

REV. 0

–9–

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