THS1215_15 Datasheet, PDF (13/22 Pages) Texas Instruments – 3.3-V, 12-BIT, 15 MSPS, LOW-POWER ANALOG-TO-DIGITAL

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Not Recommended For New Designs
THS1215
SLAS292A – MARCH 2001 – REVISED MARCH 2004
VS+
VCM
+
_
RS
RS
VS −
THS1215
RSW
RSW
CI
CI
VCM
+
_
Figure 16. Simplified Equivalent Input Circuit
To maintain the frequency performance outlined in the specifications, the total source impedance should be
limited to the following equation with f_CLK = 15 MHz, C_I = 5 pF, R_SW = 200 Ω:
RS t _2fCLK
1
CI
In(256) ^–RSW
So, for applications running at a lower f_CLK, the total source resistance can increase proportionally.
The analog input of the THS1215 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
−
+
THS1215
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_IN+ and V_IN- 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 THS1215. Assume AV_DD
13