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LTC2320-12

Rev B

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APPLICATIONS INFORMATION

INPUT DRIVE CIRCUITS

A low impedance source can directly drive the high im-

pedance inputs of the LTC2320-12 without gain error. A

high impedance source should be buffered to minimize

settling time during acquisition and to optimize the dis-

tortion performance of the ADC. Minimizing settling time

is important even for DC inputs, because the ADC inputs

draw a current spike when during acquisition.

For best performance, a buffer amplifier should be used to

drive the analog inputs of the LTC2320-12. The amplifier

provides low output impedance to minimize gain error

and allows for fast settling of the analog signal during

the acquisition phase. It also provides isolation between

the signal source and the ADC inputs, which draw a small

current spike during acquisition.

Input Filtering

The noise and distortion of the buffer amplifier and signal

sourcemustbeconsideredsincetheyaddtotheADCnoise

and distortion. Noisy input signals should be filtered prior

to the buffer amplifier input with a low bandwidth filter to

minimizenoise.Thesimple1-poleRClowpassfiltershown

in Figure 12 is sufficient for many applications.

The 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 degradation of the SNR over this bandwidth.

Figure 12. Input Signal Chain

SINGLE-ENDED

INPUT SIGNAL

232012 F12

BW = 1MHz

3.3nF

SINGLE-ENDED

TO DIFFERENTIAL

DRIVER

IN+

IN–

LTC2320

Figure 11. Fully-Differential Transfer Function

232012 F11

–VREF

VREF

AIN

(AINn+ – AINn–)

ADC CODE

(2’s COMPLEMENT)

–VREF/2

VREF/2

–2048

2047

–4096

4095

High quality capacitors and resistors should be used in the

RCfilterssincethesecomponentscanadddistortion.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

occurduringsoldering.Metalfilmsurfacemountresistors

are much less susceptible to both problems.

ADC REFERENCE

Internal Reference

The LTC2320-12 has an on-chip, low noise, low

drift (20ppm/°C max), temperature compensated band-

gap reference. It is internally buffered and is available

at REF (Pin 8). The reference buffer gains the internal

reference voltage to 4.096V for supply voltages VDD = 5V

and to 2.048V for VDD = 3.3V. The REF pin also drives

the four internal reference buffers with a current limited

output (250μA) so it may be easily overdriven with an

external reference in the range of 1.25V to 5V. Bypass

REF to GND with a 1μF (X5R, 0805 size) ceramic capacitor

to compensate the reference buffer and minimize noise.

The 1μF capacitor should be as close as possible to the

LTC2320-12 package to minimize wiring inductance. The

voltage on the REF pin must be externally buffered if used

for external circuitry.

External Reference

The internal REFOUT1,2,3,4 buffers can also be over-

driven from 1.25V to 5V with an external reference at

REFOUT1,2,3,4 as shown in Figure 13 (c). To do so,

REFBUFEN must be grounded to disable the REF buffers.

A 55k internal resistance loads the REFOUT1,2,3,4 pins

when the REF buffers are disabled. To maximize the input

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