LTC2756 datasheet(28/40 Pages) LINER

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LTC2345-18

234518f

For more information www.linear.com/LTC2345-18

in Figure 11c. 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 93dB when paired with the

LTC2345-18.BypasstheLTC6655-5toGND(Pin20)close

to the REFBUF pin with at least a 47μF ceramic capacitor

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

transient conversion currents and minimize noise.

applicaTions inForMaTion

234518 F11c

47µF

6.5k

20k

LTC2345-18

REFERENCE

BUFFER

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

LTC6655-5

Figure 11c. External Reference with Disabled Internal

Buffer Configuration

CNV

IDLE

PERIOD

IDLE

PERIOD

234518 F12

Figure 12. CNV Waveform Showing Burst Sampling

The LTC2345-18 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 12, 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

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