LTC2217CUP datasheet(21/32 Pages) LINER

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LTC2217

2217f

The internal programmable gain ampliﬁer provides the

internal reference voltage for the ADC. This ampliﬁer has

very stringent settling requirements and therefore is not

accessible for external use.

The SENSE pin can be driven ±5% around the nominal 2.5V

or 1.25V external reference inputs. This adjustment range

can be used to trim the ADC gain error or other system

gain errors. When selecting the internal reference, the

SENSE pin should be tied to VDD as close to the converter

as possible. If the sense pin is driven externally it should

be bypassed to ground as close to the device as possible

with 1μF ceramic capacitor.

Figure 7. A 2.75V Range ADC with

an External 2.5V Reference

VCM

SENSE

1.575V

3.3V

2.2

μF

2.2

μF

2217 F07

LTC1461-2.5

LTC2217

3. If the ADC is clocked with a ﬁxed-frequency sinusoidal

signal, ﬁlter the encode signal to reduce wideband

noise.

4. Balance the capacitance and series resistance at both

encode inputs such that any coupled noise will appear

at both inputs as common mode noise.

The encode inputs have a common mode range of 1.2V

to VDD. Each input may be driven from ground to VDD for

single-ended drive.

Driving the Encode Inputs

The noise performance of the LTC2217 can depend on

the encode signal quality as much as on the analog input.

The encode inputs are intended to be driven differentially,

primarily for noise immunity from common mode noise

sources. Each input is biased through a 6k resistor to a

1.6V bias. The bias resistors set the DC operating point

for transformer coupled drive circuits and can set the logic

threshold for single-ended drive circuits.

Any noise present on the encode signal will result in ad-

ditional aperture jitter that will be RMS summed with the

inherent ADC aperture jitter.

In applications where jitter is critical (high input frequen-

cies), take the following into consideration:

1. Differential drive should be used.

2. Use as large an amplitude possible. If using trans-

former coupling, use a higher turns ratio to increase the

amplitude.

Figure 8a. Equivalent Encode Input Circuit

VDD

LTC2217

2217 F08a

VDD

ENC–

ENC+

1.6V

TO INTERNAL

ADC CLOCK

DRIVERS

VDD

Figure 8b. Balun-Driven Encode

100

8.2pF

0.1

μF

0.1

μF

0.1

μF

T1 = MA/COM ETC1-1-13

RESISTORS AND CAPACITORS

ARE 0402 PACKAGE SIZE

LTC2217

2217 F08b

ENC–

ENC+

APPLICATIONS INFORMATION

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