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參數(shù)資料
| 型號： | AD7880CR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | LC2MOS Single +5 V Supply, Low Power, 12-Bit Sampling ADC |
| 中文描述： | 1-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, PARALLEL ACCESS, PDSO24 |
| 封裝： | SOIC-24 |
| 文件頁數(shù)： | 7/16頁 |
| 文件大小： | 337K |
| 代理商： | AD7880CR |
AD7880
REV. 0
–7–
+
–
V
1
R1
10 k
V
INA
AGND
AD7880*
R2
500
R3
10 k
R5
10 k
R4
10 k
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 11. Offset and Full-Scale Adjust Circuit
Unipolar Adjustments
In the case of the 0 V to 5 V unipolar input configuration, unipolar
offset error must be adjusted before full-scale error. Adjustment is
achieved by trimming the offset of the op amp driving the ana-
log input of the AD7880. T his is done by applying an input
voltage of 0.61 mV (1/2 LSB) to V
1
in Figure 11 and adjusting
the op amp offset voltage until the ADC output code flickers
between 0000 0000 0000 and 0000 0000 0001. For full-scale
adjustment, an input voltage of 4.9982 V (FS–3/2 LSBs) is
applied to V
1
and R2 is adjusted until the output code flickers
between 1111 1111 1110 and 1111 1111 1111.
T he same procedure is required for the 0 V to 10 V input con-
figuration of Figure 6. An input voltage of 1.22 mV (1/2 LSB) is
applied to V
1
in Figure 11 and the op amp’s offset voltage is
adjusted until the ADC output code flickers between 0000 0000
0000 and 0000 0000 0001. For full-scale adjustment, an input
voltage of 9.9963 V (FS–3/2 LSBs) is applied to V
1
and R2 is
adjusted until the output code flickers between 1111 1111 1110
and 1111 1111 1111.
Bipolar Adjustments
Bipolar zero and full-scale errors for the bipolar input configura-
tion of Figure 7 are adjusted in a similar fashion to the unipolar
case. Again, bipolar zero error must be adjusted before full-scale
error. Bipolar zero error adjustment is achieved by trimming the
offset of the op amp driving the analog input of the AD7880
while the input voltage is 1/2 LSB below ground. T his is done
by applying an input voltage of –1.22 mV (1/2 LSB) to V
1
in
Figure 11 and adjusting the op amp offset voltage until the
ADC output code flickers between 0111 1111 1111 and 1000
0000 0000. For full-scale adjustment, an input voltage of
4.9982 V (FS/2–3/2 LSBs) is applied to V
1
and R2 is adjusted
until the output code flickers between 1111 1111 1110 and
1111 1111 1111.
DY NAMIC SPE CIFICAT IONS
T he AD7880 is specified and tested for dynamic performance
specifications as well as traditional dc specifications such as
integral and differential nonlinearity. T he ac specifications are
required for signal processing applications such as speech recog-
nition, spectrum analysis and high speed modems. T hese appli-
cations require information on the ADC’s effect on the spectral
content of the input signal. Hence, the parameters for which the
AD7880 is specified include SNR, harmonic distortion, inter-
modulation distortion and peak harmonics. T hese terms are dis-
cussed 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. T he 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. T he theoretical signal to noise ratio for a sine wave input
is given by:
SNR =
(6.02
N +
1.76)
dB
where
N
is the number of bits.
T hus for an ideal 12-bit converter, SNR = 74 dB.
T he output spectrum from the ADC is evaluated by applying a
sine wave signal of very low distortion to the V
IN
input which is
sampled at a 66 kHz sampling rate. A Fast Fourier T ransform
(FFT ) plot is generated from which the SNR data can be ob-
tained. Figure 12 shows a typical 2048 point FFT plot of the
AD7880 with an input signal of 2.5 kHz and a sampling fre-
quency of 61 kHz. T he SNR obtained from this graph is 73dB.
It should be noted that the harmonics are taken into account
when calculating the SNR.
(1)
Figure 12. FFT Plot
E ffective Number of Bits
T he 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).
N
=
SNR
1.76
6.02
(2)
T he effective number of bits for a device can be calculated
directly from its measured SNR.
Figure 13 shows a plot of effective number of bits versus input
frequency for an AD7880 with a sampling frequency of 61 kHz.
T he effective number of bits typically remains better than 11.5
for frequencies up to 12 kHz.
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