LTC2668 datasheet(27/40 Pages) LINER

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LTC2348-16

234816fa

For more information www.linear.com/LTC2348-16

and 2ppm/°C (maximum) temperature coefficient for high

precision applications. The LTC6655-2.048 is fully speci-

fied over the H-grade temperature range, complementing

the extended temperature range of the LTC2348-16 up to

125°C.BypassingtheLTC6655-2.048witha2.7µFto100µF

ceramic capacitor close to the REFIN pin is recommended.

External Reference with Disabled Internal Buffer

The internal reference buffer supports VREFBUF = 4.4V

maximum. By grounding REFIN, the internal buffer may

be disabled allowing REFBUF to be overdriven with an

external reference voltage between 2.5V and 5V, as shown

in Figure 10c. Maximum input signal swing and SNR are

achieved by overdriving REFBUF using an external 5V

reference. The buffer feedback resistors load the REFBUF

pin with 13kΩ even when the reference buffer is disabled.

The LTC6655-5 offers the same small size, accuracy, drift,

and extended temperature range as the LTC6655-2.048,

and achieves a typical SNR of 94.9dB when paired with

the LTC2348-16. Bypass the LTC6655-5 to GND (Pin 20)

close to the REFBUF pin with at least a 47μF ceramic ca-

pacitor (X7R, 10V, 1210 size or X5R, 10V, 0805 size) to

absorb transient conversion currents and minimize noise.

The LTC2348-16 converter draws a charge (QCONV) from

the REFBUF pin during each conversion cycle. On short

time scales most of this charge is supplied by the external

REFBUF bypass capacitor, but on longer time scales all of

the charge is supplied by either the reference buffer, or

when the internal reference buffer is disabled, the external

reference. This charge draw corresponds to a DC current

equivalentofIREFBUF=QCONV • fSMPL,whichisproportional

to sample rate. In applications where a burst of samples

is taken after idling for long periods of time, as shown in

Figure 11, IREFBUF quickly transitions from approximately

0.4mA to 1.5mA (VREFBUF = 5V, fSMPL = 200kHz). This

current step triggers a transient response in the external

reference that must be considered, since any deviation in

VREFBUFaffectsconverteraccuracy.Ifanexternalreference

is used to overdrive REFBUF, the fast settling LTC6655

family of references is recommended.

Internal Reference Buffer Transient Response

Foroptimumperformanceinapplicationsemployingburst

sampling, the external reference with internal reference

bufferconfigurationshouldbeused.Theinternalreference

buffer incorporates a proprietary design that minimizes

movements in VREFBUF when responding to a burst of

conversions following an idle period. Figure 12 compares

applicaTions inForMaTion

CNV

IDLE

PERIOD

IDLE

PERIOD

234816 F11

Figure 11. CNV Waveform Showing Burst Sampling

234816 F10c

47µF

6.5k

20k

LTC2348-16

REFERENCE

BUFFER

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

LTC6655-5

Figure 10c. External Reference with Disabled

Internal Buffer Configuration

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