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LTC2208CUP-PBF

16-Bit, 130Msps ADC

Linear Technology 

LTC2208

APPLICATIONS INFORMATION

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.

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.

1.25V

VCM

2.2μF

3.3V

1μF

2 LTC1461-2.5 6

4

SENSE LTC2208

2.2μF

2208 F07

Figure 7. A 2.25V Range ADC with

an External 2.5V Reference

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 applica-

tions 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 LTC2208 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.

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, 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.

LTC2208

VDD

ENC+

ENC–

VDD

1.6V

6k

VDD 1.6V

6k

TO INTERNAL

ADC CLOCK

DRIVERS

2208 F08a

Figure 8a. Equivalent Encode Input Circuit

0.1μF T1

50Ω

0.1μF

50Ω

ENC+

LTC2208

100Ω

8.2pF

0.1μF

ENC–

T1 = MA/COM ETC1-1-13

RESISTORS AND CAPACITORS

ARE 0402 PACKAGE SIZE

2208 F08b

Figure 8b. Transformer Driven Encode

2208fc

21

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