LTC2323-14_15 datasheet(11/24 Pages) LINER

Part #	LTC2323-14

Description	Dual, 14-Bit Sign, 5Msps Differential Input ADC with Wide Input Common Mode Range

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LTC2323-14

11

232314f

For more information http://www.linear.com/LTC2323-14

APPLICATIONS INFORMATION

Figure 3. The Equivalent Circuit for the Differential

Analog Input of the LTC2323-14

Table 1: Code Ranges for the Analog Input Operational Modes

MODE

Span (VIN+ – VIN–) Min Code

Max Code

Fully Differential

–REFOUT to

+REFOUT

100 0000 0000

0000

011 1111 1111

1111

Pseudo-Differential

Bipolar

–REFOUT/2 to

+REFOUT/2

110 0000 0000

0000

001 1111 1111

1111

Pseudo-Differential

Unipolar

0 to REFOUT

000 0000 0000

0000

011 1111 1111

1111

Analog Input

The differential inputs of the LTC2323-14 provide great

flexibility to convert a wide variety of analog signals with

no configuration required. The LTC2323-14 digitizes the

difference voltage between the AIN+ and AIN– pins while

supporting a wide common mode input range. The analog

input signals can have an arbitrary relationship to each

other, provided that they remain between VDD and GND.

The LTC2323-14 can also digitize more limited classes of

analog input signals such as pseudo-differential unipolar/

bipolarandfullydifferentialwithnoconfigurationrequired.

The analog inputs of the LTC2323-14 can be modeled

by the equivalent circuit shown in Figure 3. The back-to-

back diodes at the inputs form clamps that provide ESD

protection. In the acquisition phase, 10pF (CIN) from the

sampling capacitor in series with approximately 15Ω

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

connected to the input. Any unwanted signal that is com-

mon to both inputs will be reduced by the common mode

rejection of the ADC sampler. The inputs of the ADC core

draw a small current spike while charging the CIN capaci-

tors during acquisition.

Single-Ended Signals

Single-ended signals can be directly digitized by the

LTC2323-14. These signals should be sensed pseudo-

differentially for improved common mode rejection. By

connecting the reference signal (e.g., ground sense) of

the main analog signal to the other AIN pin, any noise or

disturbance common to the two signals will be rejected

by the high CMRR of the ADC. The LTC2323-14 flexibility

handles both pseudo-differential unipolar and bipolar sig-

nals, with no configuration required. The wide common

mode input range relaxes the accuracy requirements of

any signal conditioning circuits prior to the analog inputs.

Pseudo-Differential Bipolar Input Range

The pseudo-differential bipolar configuration represents

driving one of the analog inputs at a fixed voltage, typically

VREF/2, and applying a signal to the other AIN pin. In this

case the analog input swings symmetrically around the

fixedinputyieldingbipolartwo’scomplementoutputcodes

with an ADC span of half of full-scale. This configuration

is illustrated in Figure 4, and the corresponding transfer

function in Figure 5. The fixed analog input pin need not

be set at VREF/2, but at some point within the VDD rails

allowingthealternateinputtoswingsymmetricallyaround

thisvoltage.Iftheinputsignal(AIN+–AIN–)swingsbeyond

±REFOUT/2, valid codes will be generated by the ADC and

must be clamped by the user, if necessary.

RON

15

RON

15

BIAS

VOLTAGE

232314 F03

CIN

10pF

VDD

CIN

10pF

VDD

AIN1–

AIN1+

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