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LTC1090

1090fc

6. Reduced Reference Operation

The effective resolution of the LTC1090 can be increased

by reducing the input span of the converter. The LTC1090

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. Conversion speed (ACLK frequency)

2. Offset

3. Noise

Conversion Speed with Reduced VREF

With reduced reference voltages, the LSB step size is

reduced and the LTC1090 internal comparator overdrive is

reduced. With less overdrive, more time is required to

perform a conversion. Therefore, the maximum ACLK

frequency should be reduced when low values of VREF are

used. This is shown in the typical curve of Maximum

Conversion Clock Rate vs Reference Voltage.

Offset with Reduced VREF

The offset of the LTC1090 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 0.5mV which is 0.1LSB with a 5V reference

becomes 0.5LSB with a 1V reference and 2.5LSBs with a

0.2V reference. If this offset is unacceptable, it can be

corrected digitally by the receiving system or by offsetting

the “–” input to the LTC1090.

Noise with Reduced VREF

The total input referred noise of the LTC1090 can be

reduced to approximately 200µV 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 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 200µV of noise.

For operation with a 5V reference, the 200µV noise is only

0.04LSB peak-to-peak. In this case, the LTC1090 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 1V reference, this

same 200µV noise is 0.2LSB peak-to-peak. This will

reduce the range of input voltages over which a stable

output code can be achieved by 0.2LSB. If the reference is

further reduced to 200mV, the 200µV noise becomes

equal to one LSB and a stable code may be difficult to

achieve. 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: 1µs/DIV

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