AD7890
Rev. C | Page 24 of 28
8000
7000
6000
5000
4000
3000
2000
1000
0
(X–4) (X–3) (X–2) (X–1) X (X+1) (X+2) (X+3) (X+4)
OCCURRENCES OF CODE
CODE
SAMPLING
FREQUENCY = 102.4kHz
T
A
= 25°C
01357-021
Figure 21. Histogram of 8192 Conversions with Write During Conversion
DYNAMIC PERFORMANCE
The AD7890 contains an on-chip track/hold, allowing the part
to sample input signals up to 50 kHz on any of its input
channels. Many AD7890 applications simply require it to
sequence through low frequency input signals across its eight
channels. There may be some applications, however, for which
the dynamic performance of the converter out to 40 kHz input
frequency is of interest. For these wider band sampling
applications, it is recommended that the hardware conversion
start method is used.
These applications require information on the ADC’s effect on
the spectral content of the input signal. Signal to (noise +
distortion), total harmonic distortion, peak harmonic or
spurious and intermodulation distortion are all specified.
Figure 22 shows a typical FFT plot of a 10 kHz, 0 V to 2.5 V
input after being digitized by the AD7890-2 operating at a
102.4 kHz sampling rate. The signal to (noise + distortion) is
71.5 dB and the total harmonic distortion is −85 dB. Note that
reading data from the part during conversion at 10 MHz serial
clock does have a significant impact on dynamic performance.
For sampling applications, it is therefore recommended not to
read data during conversion.
0 25.6 51.2
SIGNAL AMPLITUDE (dB)
FREQUENCY (kHz)
120
90
60
30
0
SAMPLE RATE = 102.4kHz
INPUT FREQUENCY = 10kHz
SNR = 71.5dB
T
A
= 25°C
01357-022
Figure 22. AD7890 FFT Plot
EFFECTIVE NUMBER OF BITS
The formula for signal to (noise + distortion) ratio (see the
Terminology section) is related to the resolution or number of
bits in the converter. Rewriting the formula provides 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 23
shows a typical plot of effective number of bits versus frequency
for the AD7890-2 from dc to 40 kHz. The sampling frequency is
102.4 kHz. The plot shows that the AD7890 converts an input
sine wave of 40 kHz to an effective numbers of bits of 11 which
equates to a signal to (noise + distortion) level of 68 dB.
INPUT FREQUENCY (kHz)
0
EFFECTIVE NUMBER OF BITS
12.0
11.5
11.0
10.5
10.0
4020
01357-023
Figure 23. Effective Number of Bits vs. Frequency
