AD7870LP datasheet(16/28 Pages) AD

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AD7870/AD7875/AD7876

Rev. C | Page 16 of

TRACK-AND-HOLD

GOES INTO HOLD

TRACK-AND-HOLD RETURNS TO TRACK

AND ACQUISITION TIME BEGINS

THREE-STATE

VALID

DATA

DB11 TO DB0

BUSY

DATA

t15

t17

t16

tCONVERT

t18

Figure 16. Mode 2 Timing Diagram, 12-Bit Parallel Read

BUSY

SSTRB2

DATA

SCLK3

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

HBEN1

SDATA2

1TIMES

t15, t16, AND t20 ARE THE SAME FOR A HIGH BYTE READ AS FOR A LOW BYTE READ.

2EXTERNAL 4.7kΩ PULL-UP RESISTOR.

3EXTERNAL 2kΩ PULL-UP RESISTOR;

CONTINUOUS SCLK (DASHED LINE) WHEN 12/8/CLK = –5V;

NONCONTINUOUS WHEN 12/8/CLK = 0V.

DB11 DB10

SERIAL DATA

DB0

t18

t20

t17

t11

t10

t16

t15

t19

tCONVERT

t13

t14

t12

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 VIN

input which is sampled at a 100 kHz sampling rate. A fast

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