LTC2209 datasheet(21/32 Pages) LINER

21 / 32 page

LTC2209

2209fb

The internal programmable gain amplifier provides the

internal reference voltage for the ADC. This amplifier has

very stringent settling requirements and is not accessible

for external use.

TheSENSEpincanbedriven±5%aroundthenominal2.5V

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 2.25V Range ADC with

an External 2.5V Reference

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.

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

signal,filtertheencodesignaltoreducewidebandnoise.

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.

PGA Pin

The PGA pin selects between two gain settings for the

ADC front-end. PGA = 0 selects an input range of 2.25VP-

P; PGA = 1 selects an input range of 1.5VP-P. The 2.25V

input range has the best SNR; however, the distortion will

be higher for input frequencies above 100MHz. For ap-

plications with high input frequencies, the low input range

will have improved distortion; however, the SNR will be

1.8dB worse. See the typical performance curves section.

Driving the Encode Inputs

The noise performance of the LTC2209 can depend on

the encode signal quality as much as for 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.

Figure 8a. Equivalent Encode Input Circuit

Figure 8b. Transformer Driven Encode

applications inForMation

VCM

SENSE

1.25V

3.3V

2.2µF

1µF

2209 F07

LTC2209

LTC1461-2.5

VDD

LTC2209

2209 F08a

VDD

ENC–

ENC+

1.6V

TO INTERNAL

ADC CLOCK

DRIVERS

50Ω

100Ω

8.2pF

0.1µF

T1 = MA/COM ETC1-1-13

RESISTORS AND CAPACITORS

ARE 0402 PACKAGE SIZE

5Ω

LTC2209

2209 F08b

ENC–

ENC+

LTC2209 Datasheet(PDF) 21 Page - Linear Technology

LTC2209 Datasheet(HTML) 21 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL