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PDF AD7894 Data sheet ( Hoja de datos )
| Número de pieza | AD7894 | |
| Descripción | 5 V/ 14-Bit Serial/ 5 ms ADC in SO-8 Package | |
| Fabricantes | Analog Devices | |
| Logotipo |  |
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FEATURES
Fast 14-Bit ADC with 5 s Conversion Time
8-Lead SOIC Package
Single 5 V Supply Operation
High Speed, Easy-to-Use, Serial Interface
On-Chip Track/Hold Amplifier
Selection of Input Ranges
؎10 V for AD7894-10
؎2.5 V for AD7894-3
0 V to +2.5 V for AD7894-2
High Input Impedance
Low Power: 20 mW Typ
Pin Compatible Upgrade of 12-Bit AD7895
5 V, 14-Bit Serial, 5 s
ADC in SO-8 Package
AD7894
FUNCTIONAL BLOCK DIAGRAM
REF IN
VDD
AD7894
TRACK/
VIN
SIGNAL
SCALING*
HOLD
14-BIT
ADC
CONVST
OUTPUT
REGISTER
GENERAL DESCRIPTION
The AD7894 is a fast, 14-bit ADC that operates from a single
+5 V supply and is housed in a small 8-lead SOIC. The part
contains a 5 µs successive approximation A/D converter, an on-
chip track/hold amplifier, an on-chip clock and a high speed
serial interface.
Output data from the AD7894 is provided via a high speed,
serial interface port. This two-wire serial interface has a serial
clock input and a serial data output with the external serial clock
accessing the serial data from the part.
In addition to the traditional dc accuracy specifications such as
linearity, full-scale and offset errors, the AD7894 is also speci-
fied for dynamic performance parameters including harmonic
distortion and signal-to-noise ratio.
The part accepts an analog input range of ± 10 V (AD7894-10),
± 2.5 V (AD7894-3), 0 V to +2.5 V (AD7894-2), and operates
from a single +5 V supply consuming only 20 mW typical.
The AD7894 features a high sampling rate mode and, for low
power applications, a proprietary automatic power-down mode
where the part automatically goes into power-down once conver-
sion is complete and “wakes up” before the next conversion
cycle.
The part is available in a small outline IC (SOIC).
GND
BUSY
*AD7894-10, AD7894-3
SCLK SDATA
PRODUCT HIGHLIGHTS
1. Fast, 14-Bit ADC in 8-Lead Package
The AD7894 contains a 5␣ µs ADC, a track/hold amplifier,
control logic and a high speed serial interface, all in an 8-lead
package. This offers considerable space saving over alterna-
tive solutions.
2. Low Power, Single Supply Operation
The AD7894 operates from a single +5 V supply and con-
sumes only 20 mW. The automatic power-down mode,
where the part goes into power-down once conversion is
complete and “wakes up” before the next conversion cycle,
makes the AD7894 ideal for battery powered or portable
applications.
3. High Speed Serial Interface
The part provides high speed serial data and serial clock lines
allowing for an easy, two-wire serial interface arrangement.
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1998
1 page  
AD7894
TERMINOLOGY
Signal to (Noise + Distortion) Ratio
This is the measured ratio of signal to (noise + distortion) at the
output of the A/D converter. The signal is the rms amplitude of
the fundamental. Noise is the rms sum of all nonfundamental
signals up to half the sampling frequency (fS/2), excluding dc.
The ratio is dependent upon the number of quantization levels
in the digitization process; the more levels, the smaller the quan-
tization noise. The theoretical signal to (noise + distortion) ratio
for an ideal N-bit converter with a sine wave input is given by:
Signal to (Noise + Distortion) = (6.02␣ N + 1.76) dB
Thus for a 14-bit converter, this is 86.04 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7894, it is defined as:
THD (dB) = 20 log V22 +V32 +V42 +V52 +V62
V1
where V1 is the rms amplitude of the fundamental and V2, V3,
V4, V5 and V6 are the rms amplitudes of the second through the
sixth harmonics.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to fS/2 and excluding dc) to the rms value of the
fundamental. The value of this specification is normally deter-
mined by the largest harmonic in the spectrum, but for parts
where the harmonics are buried in the noise floor, it will be a
noise peak.
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 is equal to zero. For example, the second order
terms include (fa + fb) and (fa – fb), while the third order terms
include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb).
The AD7894 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.
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
Differential Nonlinearity
This is the difference between the measured and the ideal 1␣ LSB
change between any two adjacent codes in the ADC.
Positive Gain Error (AD7894-10)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (4 × VREF – 1 LSB) after the
Bipolar Zero Error has been adjusted out.
Positive Gain Error (AD7894-3)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (VREF – 1 LSB) after the Bipolar
Zero Error has been adjusted out.
Positive Gain Error (AD7894-2)
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (VREF – 1 LSB) after the Unipolar
Offset Error has been adjusted out.
Bipolar Zero Error (AD7894-10, AD7894-3)
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal 0 V (GND).
Unipolar Offset Error (AD7894-2)
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal 1 LSB.
Negative Gain Error (AD7894-10)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (–4 × VREF + 1 LSB) after Bipolar
Zero Error has been adjusted out.
Negative Gain Error (AD7894-3)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (– VREF + 1 LSB) after Bipolar
Zero Error has been adjusted out.
Track/Hold Acquisition Time
Track/Hold acquisition time is the time required for the output
of the track/hold amplifier to reach its final value, within
± 1/2␣ LSB, after the end of conversion (the point at which the
track/hold returns to track mode). It also applies to situations
where there is a step input change on the input voltage applied
to the VIN input of the AD7894. This means that the user must
wait for the duration of the track/hold acquisition time after the
end of conversion or after a step input change to VIN before
starting another conversion, to ensure that the part operates to
specification.
REV. 0
–5–
5 Page 
AD7894
Dynamic Performance (Mode 1 Only)
With a conversion time of 5 µs, the AD7894 is ideal for wide
bandwidth signal processing applications. These applications
require information on the ADC’s effect on the spectral con-
tent of the input signal. Signal to (Noise + Distortion), Total
Harmonic Distortion, Peak Harmonic or Spurious Noise and
Intermodulation Distortion are all specified. Figure 11 shows a
typical FFT plot of a 10 kHz, ± 10␣ V input after being digitized
by the AD7894-10 operating at a 160 kHz sampling rate. The
signal to (noise + distortion) ratio is 80.24 dB and the total
harmonic distortion is –96.35 dB.
The formula for signal to (noise + distortion) ratio (see Ter-
minology section) is related to the resolution or number of bits
in the converter. Rewriting the formula, below, gives a mea-
sure of performance expressed in effective number of bits (N):
Power Considerations
In the automatic power-down mode the part may be operated at
a sample rate that is considerably less than 160 kHz. In this
case, the power consumption will be reduced and will depend
on the sample rate. Figure 13 shows a graph of the power con-
sumption versus sampling rates from 1 Hz to 100 kHz in the
automatic power-down mode. The conditions are 5 V supply
+25°C. The SCLK pin was held low and no data was read from
the part.
100
10
N = (SNR –1.76)
6.02
where SNR is Signal to (Noise + Distortion) Ratio.
1
0
fS = 160kHz
–20
FIN = 10kHz
SNR = 80.24dB
0.1
THD = –96.35dB
1
10
100
1000
10000 100000
–40 SAMPLING FREQUENCY – Hz
Figure 13. Power vs. Sampling Rate in Automatic Power-
–60 Down Mode
–80
–100
–120
82
fS = 160kHz
FIN = 10kHz
81
–140
0
10 20 30 40 50 60
FREQUENCY – kHz
Figure 11. AD7894 FFT Plot
70
80
The effective number of bits for a device can be calculated from
its measured signal to (noise + distortion) ratio. Figure 12
shows a typical plot of effective number of bits versus frequency
for the AD7894 from dc to fSAMPLING/2. The sampling fre-
quency is 160 kHz. The plot shows that the AD7894 converts
an input sine wave of 10␣ kHz to an effective numbers of bits of
13.00, which equates to a signal to (noise + distortion) level of
80.02 dB.
14
13
12
11
10
9
10 100 1000
FREQUENCY – kHz
Figure 12. Effective Number of Bits vs. Frequency
REV. 0
–11–
80
79
78
–40
–20
0
20 40 60 80
TEMPERATURE – ؇C
Figure 14. SNR + D vs. Temperature
100
90
80
70
60
50
40
30
20
10
0
10
100
FREQUENCY – kHz
Figure 15. THD vs. Frequency
1000
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet AD7894.PDF ] | |
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