ADS7813P datasheet(16/27 Pages) TI2

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ADS7813

SBAS043C

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Keep in mind that the time period when the comparator is

most sensitive to noise is fairly small. Also, the peak portion

of the noise event produced by a digital transition is fairly

brief, as most digital signals transition in a few nanoseconds.

The subsequent noise may last for a period of time longer

than this and may induce further effects which require a

longer settling time. However, in general, the event is over

within a few tens of nanoseconds.

For the ADS7813, error correction is done when the tenth bit

is decided. During this bit decision, it is possible to correct

limited errors that may have occurred during previous bit

decisions. However, after the tenth bit, no such correction is

possible. Note that for the timing diagrams shown in Figures

2, 5, 6, 7, and 8, all external digital signals should remain

static from 8

µs after the start of a conversion until BUSY

rises. The tenth bit is decided approximately 10

µs to 11µs

into the conversion.

APPLICATIONS INFORMATION

TRANSITION NOISE

If a low-noise DC input is applied to the ADS7813 and

1,000 conversions are performed, the digital output of the

converter will vary slightly in output codes. This is true for

all 16-bit SAR converters. The Transition Noise specifica-

tion found in the Electrical Characteristics section is a

statistical figure that represents the 1 sigma (

σ) limit of these

output codes.

Using a histogram to plot the number of occurrences of each

output code, the distribution should appear bell-shaped with

the peak of the curve representing the nominal output code

for the given input voltage. The

±1σ, ±2σ, and ±3σ limits

around this nominal code should contain 68.3%, 95.5%, and

99.7%, respectively, of the conversion results. As a rough

approximation, multiplying transition noise by 6 (

±3σ) will

yield the number of unique output codes which should be

present in 1,000 conversions.

FIGURE 14. Histogram of 5,000 Conversions with Input

Grounded.

FFFEh

821

FFFFh

3291

0000h

832

0001h

0002h

0003h

FFFDh

The ADS7813 has a transition noise figure of 0.6LSB,

yielding approximately 4 different output codes for 1,000

conversions. However, since

±3σ is only 99.7%, up to three

conversions have some chance of being outside this range.

In addition, the differential linearity error of each code and

the quantization performed by the converter result in histo-

grams which can deviate from the ideal. Figure 14 shows a

histogram of 5,000 conversions from the ADS7813.

AVERAGING

The noise of the converter can be reduced by averaging

conversion results. The noise will be reduced by a factor of

√n, where n is the number of averages. For example,

averaging four conversions will reduce transition noise by

half, to 0.3LSBs. Averaging should only be used for low-

frequency signals.

For higher frequency signals, a digital filter can be used to

reduce noise. This works in a similar manner to averaging:

for every reduction in the signal bandwidth by two, the

signal-to-noise ratio will improve by 3dB.

QSPI INTERFACING

Figure 15 shows a simple interface between the ADS7813

and any queued serial peripheral interface (QSPI) equipped

microcontroller (available on several Motorola devices).

This interface assumes that the convert pulse does not

originate from the microcontroller and that the ADS7813 is

the only serial peripheral.

CONV

BUSY

DATA

DATACLK

EXT/INT

ADS7813

PCS0/SS

MOSI

SCK

QSPI

CPOL = 0 (Inactive State is LOW)

CPHA = 1 (Data valid on falling edge)

QSPI port is in slave mode.

Convert Pulse

FIGURE 15. QSPI Interface to the ADS7813.

Before enabling the QSPI interface, the microcontroller

must be configured to monitor the slave select (SS) line.

When a LOW to HIGH transition occurs (indicating the end

of a conversion), the port can be enabled. If this is not done,

the microcontroller and A/D converter may not be properly

synchronized. (The slave select line simply enables commu-

nication—it does not indicate the start or end of a serial

transfer.)

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