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LTC2321-16

232116fc

For more information www.linear.com/2321-16

applicaTions inForMaTion

Input Filtering

The noise and distortion of the buffer amplifier and signal

source must be considered since they add to the ADC

noise and distortion. Noisy input signals should be filtered

prior to the buffer amplifier input with a low bandwidth

filter to minimize noise. The simple 1-pole RC lowpass fil-

ter shown in Figure 12 is sufficient for many applications.

The input resistor divider network, sampling switch on-

resistance (RON) and the sample capacitor (CIN) form a

second lowpass filter that limits the input bandwidth to

the ADC core to 110MHz. A buffer amplifier with a low

noise density must be selected to minimize the degrada-

tion of the SNR over this bandwidth.

High quality capacitors and resistors should be used in

the RC filters since these components can add distor-

tion. NPO and silver mica type dielectric capacitors have

excellent linearity. Carbon surface mount resistors can

generate distortion from self heating and from damage

that may occur during soldering. Metal film surface mount

resistors are much less susceptible to both problems.

ADC REFERENCE

Internal Reference

The LTC2321-16 has an on-chip, low noise, low drift

(20ppm/°C max), temperature compensated bandgap

reference. It is internally buffered and is available at

REFOUT1,2 (Pins 12, 26). The reference buffer gains

the internal reference voltage to 4.096V for supply volt-

ages VDD = 5V and to 2.048V for VDD = 3.3V. Bypass

REFOUT1,2 to REFRTN1,2 with the parallel combination

of a 0.1µF (X7R, 0402 size) capacitor and a 10μF (X5R,

0805 size) ceramic capacitor to compensate the reference

buffer and minimize noise. The 0.1µF capacitor should

be as close as possible to the LTC2321-16 package to

minimize wiring inductance. Tie the REFINT pin to VDD to

enable the internal reference buffer.

Table 1. REFOUT1,2 Sources and Ranges vs VDD

VDD

REFINT

REFOUT1,2 PIN

DIFFERENTIAL

SPAN

Internal 4.096V

±4.096V

External (1.25V to 5V)

±1.25V to ±5V

3.3V

Internal 2.048V

±2.048V

3.3V

External (1.25V to 3.3V)

±1.25V to ±3.3V

SINGLE-ENDED

INPUT SIGNAL

232116 F12

BW = 1MHz

3.3nF

SINGLE-ENDED

TO DIFFERENTIAL

DRIVER

IN+

IN–

LTC2321

Figure 12. Input Signal Chain

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