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LTC2372-16

237216f

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

Figure 12c. Overdriving REFBUF Using the LTC6655-5

manyapplications.Withitssmallsize,lowpower,andhigh

accuracy, the LTC6655-2.048 is well suited for use with

the LTC2372-16 when overdriving the internal reference.

The LTC6655-2.048 offers 0.025% (max) initial accuracy

and 2ppm/°C (max) temperature coefficient for high pre-

cision applications. The LTC6655-2.048 is fully specified

over the H-grade temperature range and complements

the extended temperature range of the LTC2372-16 up to

125°C.BypassingtheLTC6655-2.048witha2.7μFto100μF

ceramic capacitor close to the REFIN pin is recommended.

External Reference Unbuffered

The internal reference buffer can also be overdriven from

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

inFigure12c.Todoso,REFINmustbegroundedtodisable

the reference buffer. A 13k resistor loads the REFBUF pin

whenthereferencebufferisdisabled.Tomaximizetheinput

signal swing and corresponding SNR, the LTC6655-5 is

recommendedwhenoverdrivingREFBUF.TheLTC6655-5

offers the same small size, accuracy, drift and extended

temperature range as the LTC6655-2.048. By using a 5V

reference, an SNR of 97dB can be achieved. Bypassing

the LTC6655-5 with a 47μF ceramic capacitor (X5R, 0805

size) close to the REFBUF pin is recommended.

applicaTions inForMaTion

Figure 13. CNV Waveform Showing Burst Sampling

TheREFBUFpinoftheLTC2372-16drawsacharge(QCONV)

from the external bypass capacitor during each conversion

cycle. If the internal reference buffer is overdriven, the

237216 F12c

47µF

6.5k

15k

LTC2372-16

REFERENCE

BUFFER

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

LTC6655-5

CNV

IDLE

PERIOD

IDLE

PERIOD

237216 F13

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 13,

IREFBUF quickly goes from approximately 380µA to a maxi-

mum of 1mA for REFBUF = 5V at 500ksps. This step in DC

current draw triggers a transient response in the external

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

buffer of the LTC2372-16 truly single-shot capable since

the first sample taken after idling will yield the same re-

sult as a sample taken after the transient response of the

internal reference buffer has settled. Figures 14a, 14b,

and 14c show the transient responses of the LTC2372-16

with the internal reference buffer and with the internal

reference buffer overdriven by the LTC6655-5, both with

a bypass capacitance of 47μF in fully differential, pseudo-

differential unipolar, and pseudo-differential bipolar input

ranges, respectively.

DYNAMIC PERFORMANCE

Fast fourier transform (FFT) techniques are used to test

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

rated throughput. By applying a low distortion sine wave

and analyzing the digital output using an FFT algorithm,

the ADC’s spectral content can be examined for frequen-

cies outside the fundamental. The LTC2372-16 provides

guaranteed tested limits for both AC distortion and noise

measurements.

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