LTC2756 datasheet(25/40 Pages) LINER

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LTC2345-18

234518f

For more information www.linear.com/LTC2345-18

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

arbitraryinputdrivecapabilityoftheLTC2345-18.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 LTC2345-18

response approaches this ideal, with 120dB of SFDR

limited by the converter's second harmonic distortion

response to the 2kHz sine wave on IN+.

The ability of the LTC2345-18 to accept arbitrary signal

swings over a wide input common mode range with high

CMRR can simplify application solutions. Figure 7 depicts

one way of using the LTC2345-18 to digitize signals of

thistype.TwochannelsoftheLTC2345-18simultaneously

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.

ThecommonmodeinputrangeoftheLTC2345-18includes

VDD, allowing the circuit shown in Figure 8a to amplify and

measure a load current (ILOAD) from a single 5V supply.

Figure 8b shows a measured transient supply current

step of an LTC3207 LED driver load. Note the LTC6252

supplies limit the usable current sense range of this circuit

to 50mA to 450mA.

Figure 9a illustrates a more general method of amplifying

an input signal. 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 ADC CMRR. Figure 9b shows measured CMRR

performance of this solution, which is competitive with

thebestcommerciallyavailableinstrumentationamplifiers.

applicaTions inForMaTion

Figure 7. Simultaneously Sense Voltage (CH0) and Current

(CH1) Over a Wide Common Mode Range

Figure 8a. Sense 50mA to 450mA Current from Single 5V

Supply with Amplification

Figure 8b. Transient Supply Current Step Measured Using

Circuit in Figure 8a Loaded with LTC3207 LED Driver

LTC2345-18

234518 F07

ONLY CHANNELS 0 AND 1 SHOWN FOR CLARITY

0.1µF

47µF

0V ≤ VS1 ≤ 5V

0V ≤ VS2 ≤ 5V

VS1 – VS2

RSENSE

ISENSE =

IN0+

IN0–

IN1+

IN1–

REFIN

REFBUF

ISENSE

RSENSE

VS2

VS1

0V TO 4.096V RANGE

TIME (µs)

0 10 20 30 40 50 60 70 80 90 100

100

120

140

160

180

200

234518 F08b

LTC2345-18

234518 F08a

ONLY CHANNEL 0 SHOWN FOR CLARITY

0.1µF

47µF

IN0+

IN0–

VDD

REFIN

REFBUF

LTC6252

ILOAD

2.49k

274

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