LT6236 datasheet(21/50 Pages) LINER

21 / 50 page

LTC2372-16

237216f

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

Figure 6. Equivalent Circuit for the MUX Analog Inputs of the

LTC2372-16 at 500ksps

Figure 7. RC Equivalent Circuit for Two MUX Analog

Input Channels

applicaTions inForMaTion

Partially Settled: In this case, the beginning of acquisition

causes a disturbance of the coupling filter, which then

begins to settle out towards the nominal input voltage.

However, acquisition ends (and the conversion begins)

before the input settles to its final value. This generally

produces a gain error, but as long as the settling is linear,

no distortion is produced. The coupling filter’s response

is affected by the amplifier’s output impedance and other

parameters. A linear settling response to fast switched-

capacitor current spikes can NOT always be assumed for

precision, low bandwidth amplifiers. The coupling filter

serves to attenuate the current spikes’ high frequency

energy before it reaches the amplifier.

Fully Averaged: Consider the case where MUXOUT+/– is

directly wired to ADCIN+/–. If the coupling filter’s capaci-

tors (CFILT) at the MUX analog inputs are much larger than

the sum of the ADC’s sample capacitors (50pF) and the

MUX’soutputsummingnodecapacitances(20pF),thenthe

sampling glitch is greatly attenuated. The driving amplifier

effectively only sees the average sampling current, which

is quite small. At 500ksps, the equivalent input resistance

is approximately 28k (as shown in Figure 6), a benign

resistive load for most precision amplifiers. However,

resistive voltage division will occur between the coupling

filter’s DC resistance and MUX’s equivalent (switched-

capacitor) input resistance, thus producing a gain error.

The first form of crosstalk is often referred to as static

crosstalk. In static crosstalk, a signal applied to an OFF

channel, VINTERFERER, couples capacitively into the input

signal path, thus corrupting the input signal of the ON

channel,VSIGNAL.Figure7showsanRCmodeloftwoMUX

input channels and the associated parasitic capacitances.

Capacitive coupling from an OFF channel into the input

signal path can occur through CSW of an OFF switch to

the MUXOUT+/– output pins or through CPIN to an adja-

cent input pin or the MUXOUT+/– output pins. Coupling

through CPIN to the MUXOUT+/– pins is the dominant

coupling mechanism that limits the crosstalk to –107dB

with a 100kHz input signal applied to an OFF CH3 or CH4.

These pins sit adjacent to the MUXOUT+ and MUXOUT–

pins, respectively.

The second form of crosstalk is referred to as adjacent

channel crosstalk, which has to do with memory from the

inputofonechannelaffectingthesampledvalueofanother

channel. In this case, CPAR at the output summing nodes

of the MUX, MUXOUT+/–, can act as memory storage

elements if not dealt with properly. The potential cross-

talk mechanism here is through charge sharing. CPAR is

charged approximately to the voltage of each channel that

is sampled. If that charge is not cleared when switching

from one channel to the next, then charge sharing between

the charge on the filter capacitor (CFILT) of one channel

will occur with the charge from another channel stored on

CPAR.TheunwantedchargefromCPARcantakealongtime

to settle out depending on the input filter bandwidth. CPAR

is discharged through a low impedance switch to ground

every conversion cycle or when a first new configuration

is programmed to mitigate this effect.

Crosstalk

Crosstalk is a typical concern in systems that employ

multiplexers. The LTC2372-16 features a low crosstalk

8-channel MUX. There are two forms of crosstalk in the

LTC2372-16 that potentially allow the signal from one

channel to corrupt the signal from another channel being

sampled.

REQ

BIAS

VOLTAGE

LTC2372-16

237216 F06

CFILT >> CTOT

CTOT = CIN + CPAR = 70pF

CFILT >> CTOT

CHY, COM

CHX

REQ =

fSMPL • CTOT

237216 TA01a

CFILT

CPAR

CH2/CH5

CH3/CH4

MUXOUT+/–

CPIN

RSW

VSIGNAL

VINTERFERER

RSW

CSW

OFF CHANNEL

ON CHANNEL

CSW

LT6236 Datasheet(PDF) 21 Page - Linear Technology

LT6236 Datasheet(HTML) 21 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Datasheet Download

Link URL