OPA350

24pF

47 F

m

ADS8317

V

REF

50W

ADS8317

www.ti.com .................................................................................................................................................... SBAS356D – JUNE 2007 – REVISED OCTOBER 2009

Noise

The equivalent input circuit for the reference voltage

is presented in Figure 43. At the same time, an

The

transition

noise

of

the

ADS8317

itself

is

equivalent capacitor of 24pF is switched. To obtain

extremely low, as shown in Figure 19 and Figure 36;

optimum performance from the ADS8317, special

it is much lower than competing A/D converters.

care must be taken in designing the interface circuit

These histograms were generated by applying a

to the reference input pin. To ensure a stable

low-noise DC input and initiating 8192 conversions.

reference voltage, a 47

μF tantalum capacitor with low

The digital output of the A/D converter varies in

ESR should be connected as close as possible to the

output code because of the internal noise of the

input pin. If a high output impedance reference

ADS8317. This variance is true for all 16-bit,

source is used, an additional operational amplifier

SAR-type A/D converters. Using a histogram to plot

with a current-limiting resistor must be placed in front

the output codes, the distribution should appear

of the capacitors.

bell-shaped

with

the

peak

of

the

bell

curve

representing the nominal code for the input value.

The ±1

σ, ±2σ, and ±3σ distributions represent 68.3%,

95.5%, and 99.7%, respectively, of all codes. The

transition noise can be calculated by dividing the

number of codes measured by 6, which yields the

±3

σ distribution, or 99.7%, of all codes. Statistically,

up to three codes could fall outside the distribution

when executing 1000 conversions. The ADS8317,

with five output codes for the ±3

σ distribution, yields

less than ±0.8LSB of transition noise. Remember that

Figure 43. Input Reference Circuit and Interface

to

achieve

this

low-noise

performance,

the

peak-to-peak noise of the input signal and reference

When the ADS8317 is in power-down mode, the input

must be less than 50

μV.

resistance of the reference pin has a value of 5G

Ω.

Because the input capacitors must be recharged

Averaging

before the next conversion starts, an operational

amplifier with good dynamic characteristics, such as

The noise of the A/D converter can be compensated

the OPA350, should be used to buffer the reference

by

averaging

the

digital

codes.

By

averaging

input.

conversion results, transition noise is reduced by a

factor of 1/

√n , where n is the number of averages.

For example, averaging four conversion results

reduces

the

transition

noise

from

±0.8LSB

to

±0.4LSB. Averaging should only be used for input

signals with frequencies near DC.

For AC signals, a digital filter can be used to

low-pass filter and decimate the output codes. This

configuration works in a similar manner to averaging;

for every decimation by 2, the signal-to-noise ratio

improves by 3dB.

Copyright © 2007–2009, Texas Instruments Incorporated

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