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Noise

OPA350

24pF

47 F

ADS8317

REF

50W

Averaging

ADS8317

SBAS356A – JUNE 2007 – REVISED SEPTEMBER 2007

The equivalent input circuit for the reference voltage

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

The

transition

noise

the

ADS8317

itself

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

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

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

Figure 43. Input Reference Circuit and Interface

±0.8LSB of transition noise. Remember that to

achieve this low-noise performance, the peak-to-peak

noise of the input signal and reference must be less

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

than 50

μV.

resistance of the reference pin has a value of 5G

Ω.

Because the input capacitors must be recharged

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

averaging

the

digital

codes.

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

±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.

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