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LTC2335-18

233518f

For more information www.linear.com/LTC2335-18

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

arbitraryinputdrivecapabilityoftheLTC2335-18.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 LTC2335-18

response, which approaches this ideal with 118dB of

SFDR limited by the converter's second harmonic distor-

tion response to the 3.1kHz sine wave on IN–.

The ability of the LTC2335-18 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 LTC2335-18 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 LTC2335-18 to sense the volt-

age 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.

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

applicaTions inForMaTion

–5V

31V

LTC2335-18

233518 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

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