Datasheet
AD7870/AD7875/AD7876
Rev. C | Page 16 of 28
CS
TRACK-AND-HOLD
GOES INTO HOLD
TRACK-AND-HOLD RETURNS TO TRACK
AND ACQUISITION TIME BEGINS
THREE-STATE
VALID
DATA
DB11 TO DB0
RD
BUSY
DATA
t
15
t
17
t
7
07730-016
t
16
t
CONVERT
t
18
Figure 16. Mode 2 Timing Diagram, 12-Bit Parallel Read
CS
RD
BUSY
SSTRB
2
DATA
SCLK
3
TRACK-AND-HOLD RETURNS TO TRACK
AND ACQUISITION TIME BEGINS
TRACK-AND-HOLD
GOES INTO HOLD
THREE-STATE
LEADING
ZEROS
VALID
DATA
DB7 TO DB0
VALID
DATA
DB11 TO DB8
HBEN
1
SDATA
2
1
TIMES
t
15
,
t
16
, AND
t
20
ARE THE SAME FOR A HIGH BYTE READ AS FOR A LOW BYTE READ.
2
EXTERNAL 4.7kΩ PULL-UP RESISTOR.
3
EXTERNAL 2kΩ PULL-UP RESISTOR;
CONTINUOUS SCLK (DASHED LINE) WHEN 12/8/CLK = –5V;
NONCONTINUOUS WHEN 12/8/CLK = 0V.
DB11 DB10
SERIAL DATA
DB0
t
8
t
2
t
18
t
20
t
7
t
6
t
7
t
6
t
17
t
11
t
10
t
16
t
15
t
19
t
CONVERT
t
13
t
14
t
12
07730-017
Figure 17. Mode 2 Timing Diagram, Byte or Serial Read
The Mode 2 timing diagram for byte and serial data is shown in
Figure 17. For a two-byte data read, the lower byte (DB0 – DB7)
has to be accessed first since HBEN must be low to start conver-
sion. The ADC behaves like slow memory for this first read,
but the second read to access the upper byte of data is a normal
read. Operation of the serial functions is identical between
Mode 1 and Mode 2. The timing diagram of Figure 17 shows
both a noncontinuously and a continuously running SCLK
(dashed line).
DYNAMIC SPECIFICATIONS
The AD7870 and AD7875 are specified and 100% tested for
dynamic performance specifications as well as traditional dc
specifications such as integral and differential nonlinearity.
Although the AD7876 is not production tested for ac
parameters, its dynamic performance is similar to the AD7870
and AD7875. The ac specifications are required for signal
processing applications such as speech recognition, spectrum
analysis and high speed modems. These applications require
information on the ADC’s effect on the spectral content of the
input signal. Thus, the parameters for which the AD7870 and
AD7875 are specified include SNR, harmonic distortion,
intermodulation 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 the sampling frequency (FS/2) excluding dc. SNR is
dependent upon 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
SNR = (6.02N + 1.76) dB (1)
where N is the number of bits. Thus for an ideal 12-bit
converter, SNR = 74 dB.
Note that a sine wave signal is of very low distortion to the V
IN
input which is sampled at a 100 kHz sampling rate. A fast