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LTC1292/LTC1297

become a larger fraction of an LSB as the size of the LSB

is reduced. The typical performance characteristics

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. Here the LTC1292/LTC1297

noise will contribute virtually no uncertainty to the

output code. 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 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. Now, averaging readings may be necessary.

This noise data was taken in a very clean test fixture.

Any setup induced noise (noise or ripple on VCC, VREF

or VIN) will add to the internal noise. The lower the

reference voltage used, the more critical it becomes to

have a noise-free setup.

Gain Error Due to Reduced VREF

The gain error of the LTC1292/LTC1297 is very good

over a wide range of reference voltages. The error

component that is seen in the typical performance

characteristics curve Change in Gain Error vs Refer-

ence Voltage is due to the voltage drop on the GND pin

from the device to the ground plane. To minimize this

error the LTC1292/LTC1297 should be soldered di-

rectly onto the PC board. The internal reference point

for VREF is tied to GND. Any voltage drop in the GND pin

will make the reference voltage, internal to the device,

less than what is applied externally (Figure 19). This

drop is typically 420

µV due to the product of the pin

APPLICATI

I FOR ATIO

SNR

= 







–.

176

602

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

example shown in Figures 20a and 20b, N = 11.8 bits

and 9.9 bits, respectively. Figure 21 shows a plot of

ENOB as a function of input frequency. The 2nd har-

monic distortion term accounts for the degradation of

the ENOB as fIN approaches fS/2.

Figure 22 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

difference frequencies of the fundamentals and prod-

ucts of the fundamentals. This is classically referred to

as intermodulation distortion (IMD).

LTC1292

LTC1297

REF+

RPIN

ICC

DAC

REF–

VREF

GND

LTC1292/7 F19

± REFERENCE

VOLTAGE

resistance (RPIN) and the LTC1292/LTC1297 supply

current. For example, with VREF = 1.25V this will result

in a gain error change of –1.0LSB from the gain error

measured with VREF = 5V.

LTC1292 AC Characteristics

Two commonly used figures of merit for specifying the

dynamic performance of the A/Ds in digital signal

processing applications are the Signal-to-Noise Ratio

(SNR) and the “Effective Number of Bits (ENOB).” SNR

is the ratio of the RMS magnitude of the fundamental to

the RMS magnitude of all the non-fundamental signals

up to the Nyquist frequency (half the sampling fre-

quency). 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 depends

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

SNR is equal to 74dB. Fast Fourier Transform (FFT)

plots of the output spectrum of the LTC1292 are shown

in Figures 20a and 20b. The input (fIN) frequencies are

1kHz and 28kHz with the sampling frequency (fS) at

58.8 kHz. The SNRs obtained from the plots are 73.0dB

and 61.5dB.

By rewriting the SNR expression it is possible to obtain

the equivalent resolution based on the SNR measure-

ment.

Figure 19. Parasitic Resistance in GND Pin

1292/7 E3

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