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LTC2335-18

233518f

For more information www.linear.com/LTC2335-18

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 LTC2335-18 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. Grounding REFIN disables the internal buffer,

allowing REFBUF to be overdriven with an external refer-

ence voltage between 2.5V and 5V, as shown in Figure

10c. Maximum input signal swing and SNR are achieved

byoverdrivingREFBUFusinganexternal5Vreference.The

buffer feedback resistors load the REFBUF pin with 13kΩ

evenwhenthereferencebufferisdisabled.TheLTC6655-5

offers the same small size, accuracy, drift, and extended

temperature range as the LTC6655-2.048, and achieves

a typical SNR of 98.3dB when paired with the LTC2335-

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

The LTC2335-18 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 11, IREFBUF quickly transitions from approximately

0.4mA to 1.1mA (VREFBUF = 5V, fSMPL = 1Msps). 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

applicaTions inForMaTion

Figure 11. CNV Waveform Showing Burst Sampling

Figure 10c. External Reference with Disabled Internal

Buffer Configuration

CNV

IDLE

PERIOD

IDLE

PERIOD

233518 F11

233518 F10c

47µF

6.5k

20k

LTC2335-18

REFERENCE

BUFFER

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

LTC6655-5

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