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LTC2372-16

237216f

For more information www.linear.com/LTC2372-16

applicaTions inForMaTion

Fully Differential Input Range

The fully differential input range provides the widest

input signal swing, configuring the ADC to digitize the

differential analog input voltage to the ADC core (ADCIN+ −

ADCIN−)providedthroughtheselectedMUXanaloginputs

over a span of ±VREFBUF. In this range, the ADCIN+ and

ADCIN− pins should be driven 180 degrees out-of-phase

with respect to each other, centered around a common

mode voltage (ADCIN+ + ADCIN−)/2 that is restricted to

(VREFBUF/2 ± 0.1V). Both the ADCIN+ and ADCIN− pins are

allowed to swing from (GND − 0.1V) to (VREFBUF + 0.1V).

Unwanted signals common to both inputs are reduced

by the CMRR of the ADC. The output data format may be

selected as straight binary or two’s complement.

Pseudo-Differential Unipolar Input Range

In the pseudo-differential unipolar input range, the ADC

digitizes the differential analog input voltage to the ADC

core (ADCIN+ − ADCIN−) provided through the selected

MUX analog inputs over a span of (0V to VREFBUF). In this

range, a single-ended unipolar input signal, driven on the

ADCIN+ pin, is measured with respect to the signal ground

reference level, driven on the ADCIN− pin. The ADCIN+

pin is allowed to swing from (GND − 0.1V) to (VREFBUF +

0.1V), while the ADCIN− pin is restricted to (GND ± 0.1V).

Unwanted signals common to both inputs are reduced by

the CMRR of the ADC. The output data format is straight

binary.

Pseudo-Differential Bipolar Input Range

In the pseudo-differential bipolar input range, the ADC

digitizes the differential analog input voltage to the ADC

core (ADCIN+ − ADCIN−) provided through the selected

MUX analog inputs over a span of (±VREFBUF/2). In this

range, a single-ended bipolar input signal, driven on the

ADCIN+ pin, is measured with respect to the signal mid-

scalereferencelevel,drivenontheADCIN−pin.TheADCIN+

pin is allowed to swing from (GND − 0.1V) to (VREFBUF

+ 0.1V), while the ADCIN− pin is restricted to (VREFBUF/2

± 0.1V). Unwanted signals common to both inputs are

reduced by the CMRR of the ADC. The output data format

is two’s complement.

INPUT DRIVE CIRCUITS

Whether MUXOUT+/− is wired directly to ADCIN+/− or

through a buffer with high input impedance, the MUX

analog inputs of the LTC2372-16 are high impedance. In

either case, a low impedance source can directly drive the

MUX analog inputs without gain error. A high impedance

source should be buffered in both cases to minimize set-

tling time during acquisition and to optimize ADC linearity.

For best performance, a buffer amplifier should be used

to drive the MUX analog inputs of the LTC2372-16 with

MUXOUT+/− wired directly to ADCIN+/−. The amplifier

provides low output impedance, which produces fast

settling of the analog signal during the acquisition phase.

It also provides isolation between the signal source and

the current spikes drawn by the MUX analog inputs when

entering acquisition.

Noise and Distortion

The noise and distortion of the buffer amplifiers and signal

sources must be considered since they add to the ADC

noise and distortion. Noisy input signals should be filtered

prior to the inputs of the buffers driving the MUX analog

inputs with an appropriate filter to minimize noise. The

simple 1-pole RC lowpass filter (LPF1) shown in Figure 5

is sufficient for many applications.

Buffer amplifiers with low noise density must be selected

to minimize SNR degradation. Coupling filter networks

(LPF2) should be placed between the buffer outputs and

MUX analog inputs to both minimize the noise contribu-

tion of the buffers and reduce disturbances reflected into

the buffer from MUX analog input sampling transients.

If a buffer amplifier is used between MUXOUT+/− and

ADCIN+/−, a coupling filter network (LPF3) should be

placed between the buffer output and ADC core analog

inputs to both minimize the noise contribution of the buf-

fer and reduce disturbances reflected into the buffer from

the ADC core analog input sampling transients. Long RC

time constants at the MUX or ADC core analog inputs will

slow down the settling of those inputs. Therefore, LPF2

and LPF3 typically require wider bandwidths than LPF1.

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