LTC1290_15 Datasheet, PDF(23/32 Page) Linear Technology – Single Chip 12-Bit Data Acquisition System

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LTC1290_15 Datasheet, PDF (23/32 Pages) Linear Technology – Single Chip 12-Bit Data Acquisition System

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LTC1290

APPLICATI S I FOR ATIO

6. Reduced Reference Operation

The effective resolution of the LTC1290 can be increased

by reducing the input span of the converter. The LTC1290

exhibits good linearity and gain over a wide range of

reference voltages (see the 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 LTC1290 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 0.1LSB with a 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 âââ input to the LTC1290.

Noise with Reduced VREF

The total input referred noise of the LTC1290 can be

reduced to approximately 200ÂµV 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 200ÂµV of noise.

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

0.16LSB peak-to-peak. In this case, the LTC1290 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 200ÂµV 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 in-

duced noise (noise or ripple on VCC, VREF, VIN or Vâ) 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.

7. LTC1290 AC Characteristics

Two commonly used figures of merit for specifying the

dynamic performance of the A/Dâs in digital signal process-

ing applications are the Signal-to-Noise Ratio (SNR) and

the âeffective number of bits (ENOB).â SNR is defined as

the ratio of the RMS magnitude of the fundamental to the

RMS magnitude of all the nonfundamental signals up to the

Nyquist frequency (half the sampling frequency). The

theoretical maximum SNR for a sine wave input is given by:

SNR = (6.02N + 1.76dB)

where N is the number of bits. Thus the SNR is a function

of the resolution of the A/D. For an ideal 12-bit A/D the SNR

is equal to 74dB. A Fast Fourier Transform(FFT) plot of the

output spectrum of the LTC1290 is shown in Figures 17a

and 17b. The input (fIN) frequencies are 1kHz and 25kHz

with the sampling frequency (fS) at 50.6kHz. The SNR

obtained from the plot are 73.25dB and 72.54dB.

Rewriting the SNR expression it is possible to obtain the

equivalent resolution based on the SNR measurement.

N = (SNR â 1.76dB)/6.02

This is the so-called effective number of bits (ENOB). For

the example shown in Figures 17a and 17b, N = 11.9 bits

and 11.8 bits, respectively. Figure 18 shows a plot of ENOB

as a function of input frequency. The curve shows the

A/Dâs ENOB remain in the range of 11.9 to 11.8 for input

frequencies up to fS/2.

Figure 19 shows an FFT plot of the output spectrum for two

tones applied to the input of the A/D. Nonlinearities in the

A/D will cause distortion products at the sum and differ-

ence frequencies of the fundamentals and products of the

fundamentals. This is classically referred to as intermod-

ulation distortion (IMD).

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