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LTC2358-16

Rev A

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APPLICATIONS INFORMATION

feature of the LTC2358-16 enables it to accept a wide

variety of signal swings, simplifying signal chain design.

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

arbitraryinputdrivecapabilityoftheLTC2358-16.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 LTC2358-16

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 LTC2358-16 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 LTC2358-16 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 LTC2358-16 to simultaneously

sensethevoltageandbidirectionalcurrentthroughasense

resistor over a wide common mode range.

Input Filtering

The true high impedance analog inputs can accommodate

a very wide range of passive or active signal conditioning

filters. The buffered ADC inputs have an analog bandwidth

of6MHz,andimposenoparticularbandwidthrequirement

onexternalfilters.Theexternalinputfilterscanthereforebe

optimized independent of the ADC to reduce signal chain

noise and interference. A common filter configuration is

thesimpleanti-aliasingandnoisereducingRCfilterwithits

pole at half the sampling frequency. For example, 100kHz

with R=2.43kΩ and C=680pF as shown in Figure 5.

–15V

15V

LTC2358-16

235816 F05

ONLY CHANNEL 0 SHOWN FOR CLARITY

TRUE BIPOLAR

+10V

–10V

+10V

–10V

UNIPOLAR

0.1µF

47µF

IN0+

IN0–

VCC

REFIN

REFBUF

VEE

OPTIONAL

LOWPASS FILTER

680pF

R = 2.43k

IN+

IN–

Figure 5. Filtering Single-Ended Input Signals

High quality capacitors and resistors should be used in

the RC filters since these components can add distortion.

NPO/COG and silver mica type dielectric capacitors have

excellent linearity. Carbon surface mount resistors can

generate distortion from self-heating and from damage

that may occur during soldering. Metal film surface mount

resistors are much less susceptible to both problems.

Arbitrary and Fully Differential Analog Input Signals

The wide common mode input range and high CMRR of

the LTC2358-16 allow each channel’s IN+ and IN– pins to

swingwithanarbitraryrelationshiptoeachother,provided

each pin remains between (VCC – 4V) and (VEE + 4V). This

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