LTC1289BCSW datasheet(20/28 Pages) LINER

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LTC1289

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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 VREF because 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

µV peak-to-peak using a

ground plane, 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.

However, 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

reduce 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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