LT1469 datasheet(12/26 Pages) LINER

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LTC2326-16

232616fa

For more information www.linear.com/LTC2326-16

applicaTions inForMaTion

Figure 4. Input Signal Chain

a total impedance of 2kΩ. The resistor divider network

attenuates and level shifts the ±2.5 • REFBUF true bipolar

signal swing of each input to the 0-REFBUF input signal

swing of the ADC core. In the acquisition phase, 45pF (CIN)

from the sampling CDAC in series with approximately 50Ω

(RON) from the on-resistance of the sampling switch is

connected to the output of the resistor divider network.

Any unwanted signal that is common to both inputs will

be reduced by the common mode rejection of the ADC

core and resistor divider network. The IN+ input of the

ADC core draws a current spike while charging the CIN

capacitor during acquisition.

INPUT DRIVE CIRCUITS

A low impedance source can directly drive the high im-

pedance input of the LTC2326-16 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 input

draws a current spike when entering acquisition.

For best performance, a buffer amplifier should be used

to drive the analog input of the LTC2326-16. 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 input which draws 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 4 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 7MHz. A buffer amplifier with a low noise

density must be selected to minimize the degradation of

the SNR over this bandwidth.

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.

Pseudo-Differential Bipolar Inputs

For most applications, we recommend the low power

LT1468 ADC driver to drive the LTC2326-16. With a low

noise density of 5nV/√Hz and a low supply current of 3mA,

the LT1468 is flexible and may be configured to convert

signals of various amplitudes to the ±10.24V input range

of the LTC2326-16.

To achieve the full distortion performance of the

LTC2326-16, a low distortion single-ended signal source

driven through the LT1468 configured as a unity-gain

buffer as shown in Figure 4 can be used to get the full

data sheet THD specification of –111dB.

ADC REFERENCE

There are three ways of providing the ADC reference. The

first is to use both the internal reference and reference

buffer. The second is to externally overdrive the internal

reference and use the internal reference buffer. The third

is to disable the internal reference buffer and overdrive

the REFBUF pin from an external source. The following

tables give examples of these cases and the resulting

bipolar input ranges.

66nF

BW = 48kHz

±10.24V

–

LT1468

LTC2326-16

IN+

IN–

232616 F04

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