THS1230CPWR datasheet(14/22 Pages) TI

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VIN+

VIN−

−

THS1230

AIN+

AIN−

REFT

REFB

AVDD

THS1230

SLAS291B – OCTOBER 2000 – REVISED MARCH 2004

The analog input of the THS1230 is a differential input that can be configured in various ways depending on the

signal source and the required level of performance. A fully differential connection (see Figure 17) delivers the

best performance from the converter.

Figure 17. AC-Coupled Differential Input

The analog input can be dc-coupled (see Figure 18) as long as the inputs are within the analog input common

mode voltage range. For example (see Figure 18), V+ and V– are signals centered on GND with a peak-to-peak

voltage of 2 V, and the circuit in Figure 18 is used to interface it with the THS1230. Assume AVDD of the

converter is 3 V. Two problems have to be solved. The first is to shift common mode level (CML) from 0 V to 1.5

V (AVDD/2). To do that, a V bias voltage and an adequate ratio of R1 and R2 have to be selected. For instance, if

V bias = AVDD = 3 V, then R1 = R2. The second is that the differential voltage has to be reduced from 4 V (2 x 2

V) to 1 V, and for that an attenuation of 4 to1 is needed. The attenuation is determined by the relation:

(R3||2R2)/((R3||2R2) + 2R1). One possible solution is R1 = R2 = R3 = 150

Ω. In this case, moreover, the input

impedance (2R1 + (R3||2R2)) will be 400

Ω. The values can be changed to match any other input impedance. A

capacitor, C, connected from AIN+ to AIN– helps filter any high frequency noise on the inputs, also improving

performance. Note that the chosen value of capacitor C must take into account the highest frequency component

of the analog input signal.

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