LT6236 datasheet(20/50 Pages) LINER

20 / 50 page

LTC2372-16

237216f

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

applicaTions inForMaTion

Figure 5. Input Signal Chain

coupling filters that are used to both filter noise and re-

duce sampling transients due to the current spikes.

The MUX and ADC core analog inputs may be modeled

as a switched capacitor load on the drive circuit. A drive

circuitmayrelypartiallyonattenuatingswitched-capacitor

current spikes with small filter capacitors CFILT placed

directlyattheADCinputsandpartiallyonthedriveramplifier

having sufficient bandwidth to recover from the residual

disturbance.AmplifiersoptimizedforDCperformancemay

not have sufficient bandwidth to fully recover at the ADC’s

maximumconversionrate,whichcanproducenonlinearity

and other errors. Coupling filter circuits may be classified

in three broad categories:

Fully Settled: This case is characterized by filter time

constants and an overall settling time that are consider-

ably shorter than the sample period. When acquisition

begins, the coupling filter is disturbed. For a typical first

order RC filter, the disturbance will look like an initial step

with an exponential decay. The amplifier will have its own

response to the disturbance, which may include ringing. If

the input settles completely (to within the accuracy of the

LTC2372-16),thedisturbancewillnotcontributeanyerror.

Table 1 lists typical recommended values for the R and C

of each LPF mentioned.

Table 1. Recommended R and C Values for Each Lowpass Filter

Rx(Ω)

Cx(pF)

BANDWIDTH

LPF1

100000

31.8kHz

LPF2

1200

13MHz

LPF3

2700

2.4MHz

High quality capacitors and resistors should be used in the

RCfilterssincethesecomponentscanadddistortion.NPO

and silver mica type dielectric capacitors have excellent

linearity. Carbon surface mount resistors can generate

distortion from self heating and from damage that may

occurduringsoldering. Metalfilmsurfacemountresistors

are much less susceptible to both problems.

Input Currents

One of the biggest challenges in coupling an amplifier to

the LTC2372-16 is in dealing with current spikes drawn

by the MUX and ADC core analog inputs at the start of

each acquisition phase. LPF2 and LPF3 are examples of

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

MUXOUT+/–

BANDLIMITING

SIGNAL SOURCE

NOISE

BANDLIMITING

BUFFER NOISE

AND REDUCING

SAMPLING TRANSIENTS

BANDLIMITING

BUFFER NOISE

AND REDUCING

SAMPLING TRANSIENTS

ADCIN+/–

LPF2

1/2 LPF2

LTC2372-16

16-BIT ADC CORE

–

237216 F05

SIGNAL

SOURCES

LPF1

1/2 LPF1

LPF3

LPFx

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