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LTC1285/LTC1288

APPLICATION INFORMATION

FREQUENCY (kHz)

– 120

– 100

–80

–60

–40

1.0

2.0

3.0

4.0

LTC1285/88 • TPC16

–20

0.5

1.5

2.5

3.5

TA = 25°C

VCC = 2.7V

VREF = 2.5V

fIN = 3.05kHz

fCLK = 120kHz

fSMPL = 7.5kHz

Figure 11. LTC1285 Non-Averaged, 4096 Point FFT Plot

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

3. Conversion speed (CLK frequency)

Offset with Reduced VREF

The offset of the LTC1285 has a larger effect on the output

code. When the ADC 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 Change in Offset vs

Reference Voltage shows how offset in LSBs is related to

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

a VOS of 122µV which is 0.2LSB with a 2.5V reference

becomes 1LSB with a 1V reference and 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 of the LTC1285.

Noise with Reduced VREF

The total input referred noise of the LTC1285 can be

reduced to approximately 400

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.

For operation with a 2.5V reference, the 400

µV noise is

only 0.66LSB peak-to-peak. In this case, the LTC1285

noise will contribute a little bit of 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 400

µV noise is 1.32LSB

peak-to-peak. This will reduce the range of input volt-

ages over which a stable output code can be achieved by

1LSB. If the reference is further reduced to 1V, the 400

µV

noise becomes equal to 3.3LSBs and a stable code may

be difficult to achieve. In this case averaging multiple

readings may be necessary.

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

induced noise (noise or ripple on VCC, VREF or VIN) 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.

Conversion Speed with Reduced VREF

With reduced reference voltages, the LSB step size is

reduced and the LTC1285 internal comparator over-

drive is reduced. Therefore, it may be necessary to

reduce the maximum CLK frequency when low values

of VREF are used.

DYNAMIC PERFORMANCE

The LTC1285/LTC1288 have exceptional sampling capa-

bility. Fast Fourier Transform (FFT) test techniques are

used to characterize the ADC’s frequency response, dis-

tortion and noise at the rated throughput. By applying a

low distortion sine wave and analyzing the digital output

using an FFT algorithm, the ADC’s spectral content can be

examined for frequencies outside the fundamental. Figure

11 shows a typical LTC1285 plot.

LTC1288CS8 Datasheet(PDF) 18 Page - Linear Technology

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