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).
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.
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.
Figure 11. FFT Plot
REV. 0
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ADI [ ADI ]
