THS1230 Datasheet, PDF (14/22 Pages) Texas Instruments – 3.3-V, 12-BIT, 30 MSPS, LOW-POWER ANALOG-TO-DIGITAL CONVERTER WITH POWER DOWN

THS1230
SLAS291B – OCTOBER 2000 – REVISED MARCH 2004
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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.
AVDD
2
VIN+ ^C1
C2
VIN− ^C1
C2
R2
R1
−
+
R2
R1
−
+
THS1230
AIN+
AIN−
REFT
REFB
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 AV_DD 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 (AV_DD/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 = AV_DD = 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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