LTC2336-18_15 datasheet(13/24 Pages) LINER

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LTC2336-18

233618f

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applicaTions inForMaTion

Figure7b. Using the LTC6655-2.048 as an External Reference

Figure 7c. Overdriving REFBUF Using the LTC6655-5

Figure 8. CNV waveform Showing Burst Sampling

LTC2336-18

BANDGAP

REFERENCE

LTC6655-2.048

233618 F07b

47µF

2.7µF

6.5k

REFBUF

REFIN

15k

6.5k

REFERENCE

BUFFER

GND

LTC2336-18

GND

BANDGAP

REFERENCE

LTC6655-5

233618 F07c

47µF

6.5k

REFBUF

REFIN

15k

6.5k

REFERENCE

BUFFER

External Reference Unbuffered

The internal reference buffer can also be overdriven from

2.5V to 5V with an external reference at REFBUF as shown

in Figure 7c. So the input ranges are ±6.25V to ±12.5V,

respectively, as shown in Table 3. To do so, REFIN must

be grounded to disable the reference buffer. A 13k resis-

tor loads the REFBUF pin when the reference buffer is

disabled. To maximize the input signal swing and cor-

responding SNR, the LTC6655-5 is recommended when

overdrivingREFBUF.TheLTC6655-5offersthesamesmall

size, accuracy, drift and extended temperature range as

the LTC6655-2.048. By using a 5V reference, an SNR of

102dB can be achieved. Bypassing the LTC6655-5 with

a 47μF ceramic capacitor (X5R, 0805 size) close to the

REFBUF pin is recommended.

TheREFBUFpinoftheLTC2336-18drawsacharge(QCONV)

fromtheexternalbypasscapacitorduringeachconversion

cycle. If the internal reference buffer is overdriven, the

external reference must provide all of this charge with a

DC current equivalent to IREFBUF = QCONV/tCYC. Thus, the

DC current draw of REFBUF depends on the sampling rate

and output code. In applications where a burst of samples

is taken after idling for long periods, as shown in Figure 8,

IREFBUF quickly goes from approximately 390µA to a maxi-

mumof0.6mAforREFBUF=5Vat250ksps.ThisstepinDC

current draw triggers a transient response in the external

CNV

IDLE

PERIOD

IDLE

PERIOD

233618 F08

reference that must be considered since any deviation in

the voltage at REFBUF will affect the accuracy of the output

code. If an external reference is used to overdrive REFBUF,

the fast settling LTC6655-5 reference is recommended.

Internal Reference Buffer Transient Response

Foroptimumtransientperformance,theinternalreference

buffer should be used. The internal reference buffer uses a

proprietary design that results in an output voltage change

at REFBUF of less than 1LSB when responding to a sudden

burst of conversions. This makes the internal reference

bufferoftheLTC2336-18trulysingle-shotcapablesincethe

first sample taken after idling will yield the same result as

a sample taken after the transient response of the internal

reference buffer has settled. Figure 9 shows the transient

responses of the LTC2336-18 with the internal reference

buffer and with the internal reference buffer overdriven by

the LTC6655-5, both with a bypass capacitance of 47μF.

DYNAMIC PERFORMANCE

Fast Fourier Transform (FFT) techniques are used to test

the ADC’s frequency response, distortion and noise at the

ratedthroughput.Byapplyingalowdistortionsinewaveand

analyzingthedigitaloutputusinganFFTalgorithm,theADC’s

spectralcontentcanbeexaminedforfrequenciesoutsidethe

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