LTC1289CC datasheet(20/28 Pages) LINER

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LTC1289

APPLICATI

I FOR ATIO

Figure 15. Adequate Reference Settling

HORIZONTAL: 1

µs/DIV

2. It is recommended that REF– input be tied directly to

the analog ground plane. If REF– is biased at a voltage

other than ground, the voltage must not change during

a conversion cycle. This voltage must also be free of

noise and ripple with respect to analog ground.

6. Reduced Reference Operation

The effective resolution of the LTC1289 can be increased by

reducing the input span of the converter. The LTC1289

exhibits good linearity and gain over a wide range of

reference voltages (see typical curves of Linearity and Gain

Error vs Reference Voltage). However, care must be taken

when operating at low values of VREFbecause of the reduced

LSB step size and the resulting higher accuracy requirement

placed on the converter. The following factors must be

considered when operating at low VREF values:

1. Offset

2. Noise

Offset with Reduced VREF

The offset of the LTC1289 has a larger effect on the output

code when the A/D is operated with reduced reference

voltage. The offset (which is typically a fixed voltage)

becomes a larger fraction of an LSB as the size of the LSB is

reduced. The typical curve of Unadjusted Offset Error vs

Reference Voltage shows how offset in LSBs is related to

reference voltage for a typical value of VOS. For example, a

VOS of 0.1mV which is 0.2LSB with a 2.5V reference

becomes 0.4LSB with a 1.25V reference. If this offset is

unacceptable, it can be corrected digitally by the receiving

system or by offsetting the “–” input to the LTC1289.

Noise with Reduced VREF

The total input referred noise of the LTC1289 can be reduced

to approximately 200

µVpeak-to-peakusingagroundplane,

good bypassing, good layout techniques and minimizing

noise on the reference inputs. This noise is insignificant with

a 2.5V reference but will become a larger fraction of an LSB

as the size of the LSB is reduced. The typical curve of Noise

Error vs Reference Voltage shows the LSB contribution of

this 200

µV of noise.

For operation with a 2.5 reference, the 200

µV noise is only

0.32LSB peak-to-peak. In this case, the LTC1289 noise will

contribute virtually no uncertainty to the output code. How-

ever, for reduced references, the noise may become a

significant fraction of an LSB and cause undesirable jitter in

the output code. For example, with a 1.25V reference, this

same 200

µV noise is 0.64LSB peak-to-peak. This will re-

duce the range of input voltages over which a stable output

code can be achieved by 0.64LSB. In this case averaging

readings may be necessary.

This noise data was taken in a very clean setup. Any setup

induced noise (noise or ripple on VCC, VREF, VIN or V–) will

add to the internal noise. The lower the reference voltage to

be used, the more critical it becomes to have a clean, noise-

free setup.

Figure 16. Poor Reference Settling Can Cause A/D Errors

HORIZONTAL: 1

µs/DIV

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