LTC1294BCSW datasheet(22/28 Pages) LINER

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LTC1293/LTC1294/LTC1296

129346fs

LTC1293/4/6 AC Characteristics

Two commonly used figures of merit for specifying the

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

ing 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 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 depends on

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 LTC1294 is shown in Figures 16a

and 16b. The input (fIN) frequencies are 1kHz and 22kHz

with the sampling frequency (fS) at 45.4kHz. The SNR

obtained from the plot are 72.7dB and 72.5dB.

Rewriting the SNR expression it is possible to obtain the

equivalent resolution based on the SNR measurement.

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

the example shown in Figures 16a and 16b, N = 11.8 bits.

Figure 17 shows a plot of ENOB as a function of input

frequency. The top curve shows the A/D’s ENOB remains

at 11.8 for input frequencies up to fS/2 with ±5V supplies.

APPLICATI

I FOR ATIO

a 1.25 reference. If this offset is unacceptable, it can be

corrected digitally by the receiving system or by offsetting

the “–” input to the LTC1293/4/6.

Noise with Reduced VREF

The total input referred noise of the LTC1293/4/6 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 input but will become

a larger fraction of an LSB as the size of the LSB is reduced.

The typical performance characteristic 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 LTC1293/4/6 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 LTC1294/6 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 Reference Voltage for the LTC1293

is due the voltage drop on the AGND pin from the device

to the ground plane. To minimize this error the LTC1293

should be soldered directly onto the PC board. The internal

reference point for VREF is tied to AGND. Any voltage drop

in the AGND pin will make the reference voltage, internal

to the device, less than what is applied externally (Figure

15). This drop is typically 400

µV due to the product of the

pin resistance (RPIN) and the LTC1293 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.

SNR

⎛

⎝⎜

⎞

⎠⎟

–.

176

602

Figure 15. Parasitic Pin Resistance (RPIN)

LTC1293

REF+

RPIN

ICC

DAC

REF–

VREF

AGND

LTC1293 F15

± REFERENCE

VOLTAGE

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