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LTC1290 Datasheet(PDF) 22 Page - Linear Technology |
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LTC1290 Datasheet(HTML) 22 Page - Linear Technology |
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22 / 28 page  22 LTC1290 S APPLICATI 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 “–” input to the LTC1290. Noise with Reduced VREF The total input referred noise of the LTC1290 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 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 5V reference, the 200 µV 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 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. 7. LTC1290 AC Characteristics Two commonly used figures of merit for specifying the dynamic performance of the A/D’s in digital signal pro- cessing applications are the Signal-to-Noise Ratio (SNR) and the “effective number of bits (ENOB).” 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 – 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/D’s 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 |
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