AD7864 Analog Devices, AD7864 Datasheet - Page 21

• Manufacturer Part Number: AD7864
• Description: High Speed, Low Power, 4-channel Simultaneous Sampling, 12-Bit ADC
• Manufacturer: Analog Devices

Specifications of AD7864

• Resolution (bits): 12bit
• # Chan: 4
• Sample Rate: 520kSPS
• Interface: Par
• Analog Input Type: SE-Bip,SE-Uni
• Ain Range: Bip 10V,Bip 2.5V,Bip 5.0V,Uni 2.5V,Uni 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

DYNAMIC SPECIFICATIONS
The AD7864 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 signal processing applications such as phased array
sonar, adaptive filters, and spectrum analysis. These applications
require information on the effect of the ADC on the spectral
content of the input signal. Thus, the parameters for which the
AD7864 is specified include SNR, harmonic distortion, inter-
modulation 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 of the sampling frequency (f
on the number of quantization levels used in the digitization
process; the more levels, the smaller the quantization noise. The
theoretical signal-to-noise ratio for a sine wave input is given by
where N is the number of bits.
Thus, for an ideal 12-bit converter, SNR = 74 dB.
Figure 16 shows a histogram plot for 8192 conversions of a dc
input using the AD7864 with a 5 V supply. The analog input was
set at the center of a code. The figure shows that all the codes
appear in the one output bin, indicating very good noise
performance from the ADC.
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 17 shows a typical 4096 point
FFT plot of the AD7864 with an input signal of 99.9 kHz and a
sampling frequency of 500 kHz. The SNR obtained from this
SNR = (6.02N + 1.76) dB
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
Figure 16. Histogram of 8192 Conversions of a DC Input
1054
ADC CODE
S
/2) excluding dc. SNR depends
Rev. D | Page 21 of 28
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graph is 72.6 dB. Note that the harmonics are taken into
account when calculating the SNR.
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).
The effective number of bits for a device can be calculated
directly from its measured SNR. Figure 18 shows a typical plot
of effective number of bits vs. frequency for an AD7864-2.
INTERMODULATION DISTORTION
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities creates distortion products
at sum and difference frequencies of mfa ± nfb where m, n = 0,
1, 2, 3, and so forth. Intermodulation terms are those for which
neither m nor n are equal to zero. For example, the second-order
N
–100
–110
12
11
10
–10
–20
–30
–40
–50
–60
–70
–80
–90
9
8
7
6
5
4
=
0
0
0
SNR
Figure 18. Effective Numbers of Bits vs. Frequency
. 6
−
02
500
. 1
76
50
1000
Figure 17. FFT Plot
FREQUENCY (kHz)
FREQUENCY (kHz)
100
1500
INPUT FREQUENCY OF 99,857Hz
+105°C
150
SAMPLING AT 499,712Hz
2000
8192 SAMPLES TAKEN
–40°C
AD7864-1 @ 25°C
200
2500
5V SUPPLY
+25°C
AD7864
3000
250
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