AD7865ASZ Datasheet, PDF (16/19 Pages) Analog Devices – Four-Channel, Simultaneous Sampling, Fast, 14-Bit ADC

AD7865
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ADC CODE
Figure 14. Histogram of 8192 Conversions of a DC Input
The output spectrum from the ADC is evaluated by applying a
sine wave signal of very low distortion to the analog input. A
Fast Fourier Transform (FFT) plot is generated from which the
SNR data can be obtained. Figure 15 shows a typical 4096-
point FFT plot of the AD7865 with an input signal of 100 kHz
and a sampling frequency of 350 kHz. The SNR obtained from
this graph is 80.5 dB. It should be noted that the harmonics are
taken into account when calculating the SNR.
0
–20
–40
–60
–80
–100
–120
fs = 350kHz
f_IN = 100kHz
SNR = 80.5dB
–140
0
35000
70000
105000
FREQUENCY – Hz
140000
175000
Figure 15. 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
obtain a measure of performance expressed in effective number
of bits (N).
N = ^{SNR −1.76}
(2)
6.02
The effective number of bits for a device can be calculated
directly from its measured SNR. Figure 16 shows a typical plot
of effective number of bits versus frequency for an AD7865-2.
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– 55؇C
12
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+25؇C
10
9
8
+125؇C
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1
0
0
100
1000
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INPUT FREQUENCY – kHz
Figure 16. Effective Numbers of Bits vs. Frequency
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products at sum and difference frequencies of mfa ± nfb where
m, n = 0, 1, 2, 3 . . ., etc. Intermodulation terms are those for
which neither m nor n are equal to zero. For example, the sec-
ond order terms include (fa + fb) and (fa – fb) while the third
order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).
The AD7865 is tested using two input frequencies. In this case
the second and third order terms are of different significance.
The second order terms are usually distanced in frequency from
the original sine waves while the third order terms are usually at
a frequency close to the input frequencies. As a result, the second
and third order terms are specified separately. The calculation of
the intermodulation distortion is as per the THD specification
where it is the ratio of the rms sum of the individual distortion
products to the rms amplitude of the fundamental expressed in
dBs. In this case, the input consists of two, equal amplitude, low
distortion sine waves. Figure 17 shows a typical IMD plot for
the AD7865.
0
fa = 49.113kHz
–20
fb = 50.183kHz
fs = 350kHz
–40
–60
–80
–100
–120
–140
0
25000 50000 75000 100000 125000 150000 175000
FREQUENCY – Hz
Figure 17. IMD Plot
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REV. B