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VIN+

VIN−

R1

R1

R3

THS1215

AIN+

REFT

REFB

AIN−

R2

R2

VBIAS

VBIAS

V IN

AVDD

2

THS1215

AIN+

AIN−

REFT

REFB

THS1215

SLAS292A – MARCH 2001 – REVISED MARCH 2004

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

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)) is 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.

Figure 18. DC-Coupled Differential Input Circuit

A single-ended source may give better overall system performance when it is converted to a differential signal

before driving the THS1215. The configuration in Figure 19 takes a VIN of 1 V and drives the 1:1 transformer

ratio so that value of AIN+ and AIN– converts to full-scale value at the ADC digital output. With VIN at –1 V the

value at AIN+ and AIN– converts to 0 at the ADC digital outputs.

Figure 19. Transformer Coupled Single-Ended Input

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