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LTC2348-16

234816fa

For more information www.linear.com/LTC2348-16

SNR and THD performance is acceptable, it is possible

to drive the LTC2348-16 using the lower-power LT1355.

The LT1355 combines fast settling, good linearity, and

moderate offset with 10nV/√Hz input-referred noise den-

sity, enabling it to drive the LTC2348-16 with only 0.1dB

SNR loss and 2dB THD loss compared with the LT1469.

As shown in Table 2, both the LT1469 and LT1355 may be

used without a lowpass filter at a loss of ≤0.5dB SNR due

to increased wideband noise. For sampling frequencies

with minimum acquisition times (tACQ) under 500ns, use

either the LT1469 or LT1355 without lowpass filtering, or

the LT1358 with lowpass filtering, for the best settling,

linearity, and THD performance.

The two-tone test shown in Figure 6b demonstrates the

arbitraryinputdrivecapabilityoftheLTC2348-16.Thistest

simultaneouslydrivesIN+witha−7dBFS2kHzsingle-ended

sine wave and IN− with a −7dBFS 3.1kHz single-ended sine

wave. Together, these signals sweep the analog inputs

across a wide range of common mode and differential

mode voltage combinations, similar to the more general

arbitrary input signal case. They also have a simple spec-

tral representation. An ideal differential converter with no

common-mode sensitivity will digitize this signal as two

−7dBFS spectral tones, one at each sine wave frequency.

The FFT plot in Figure 6b demonstrates the LTC2348-16

response approaches this ideal, with 119dB of SFDR

limited by the converter's second harmonic distortion

response to the 3.1kHz sine wave on IN–.

The ability of the LTC2348-16 to accept arbitrary signal

swings over a wide input common mode range with high

CMRR can simplify application solutions. In practice,

many sensors produce a differential sensor voltage riding

on top of a large common mode signal. Figure 7a depicts

one way of using the LTC2348-16 to digitize signals of

this type. The amplifier stage provides a differential gain

of approximately 10V/V to the desired sensor signal while

the unwanted common mode signal is attenuated by the

ADCCMRR.Thecircuitemploysthe±5VSoftSpanrangeof

the ADC. Figure 7b shows measured CMRR performance

of this solution, which is competitive with the best com-

mercially available instrumentation amplifiers. Figure 7c

shows measured AC performance of this solution.

In Figure 8, another application circuit is shown which

uses two channels of the LTC2348-16 to simultaneously

sense the voltage on and bidirectional current through a

sense resistor over a wide common mode range. In many

applications of this type, the impedance of the external

circuitry is low enough that the ADC sampling network

can fully settle without buffering.

–5V

31V

LTC2348-16

234816 F07a

ONLY CHANNEL 0 SHOWN FOR CLARITY

24V

ARBITRARY

–

0.1µF

47µF

IN0+

IN0–

½ LT1124

VCC

REFIN

REFBUF

VEE

LOWPASS FILTERS

BW ~ 500kHz

6.6nF

49.9

549

18pF

IN+

IN–

2.49k

COMMON MODE

INPUT RANGE

DIFFERENTIAL MODE

INPUT RANGE: ±500mV

Figure 7a. Digitize Differential Signals Over a Wide Common Mode Range

applicaTions inForMaTion

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