THS1240 Datasheet, PDF (8/20 Pages) Texas Instruments – 12-BIT 40 MSPS IF SAMPLING COMMUNICATIONS ANALOG-TO-DIGITAL CONVERTER

THS1240
12-BIT 40 MSPS IF SAMPLING COMMUNICATIONS
ANALOG-TO-DIGITAL CONVERTER
SLAS279D – JUNE 2000 – REVISED JANUARY 2001
APPLICATION INFORMATION
using the THS1240 references
The option of internal or external reference is provided by allowing for an external connection of the internal
reference to the reference inputs. This type of reference selection offers the lowest noise possible by not relying
on any active switch to make the selection. Compensating each reference output with a 1-µF and 0.01-µF chip
capacitor is required as shown in Figure 6. The differential analog input range is equal to 2 (V_REFOUT+ –
V_{REFOUT–)}. When using external references, it is best to decouple the reference inputs with a 0.1-µF and
0.01-µF chip capacitor as shown in Figure 7.
VREFIN+
VREFOUT+
External Reference +
VREFIN+
0.01 µF
1 µF
0.01 µF
1 µF
VREFIN_–
VREFOUT_–
0.01 µF 0.1 µF
External Reference –
0.01 µF 0.1 µF
VREFIN_–
Figure 6. Internal Reference Usage
Figure 7. External Reference Usage
using the THS1240 clock input
The THS1240 clock input can be driven with either a differential clock signal or a single ended clock input with
little or no difference in performance between the single-ended and differential-input configurations. The
common mode of the clock inputs is set internally to V_DD/2 using 5-kΩ resistors (Figure 4).
The THS1240 clock input requires a common mode voltage or dc component of V_DD/2. It is possible for the
common mode voltage of the clock source to differ from V_DD/2 by as much as 10% with little or no performance
degradation. The clock input should be either a sinewave or a square wave having a 50% duty cycle.
When driven with a single-ended CMOS clock input, it is best to connect the CLK– input to ground with a 0.01 µF
capacitor (see Figure 8).
Square Wave or Sine Wave
2 V p-p to 5 V p-p
Common Mode Voltage = VDD/2
CLK+
THS1240
CLK–
0.01 µF
Figure 8. Driving the Clock From a Single-Ended Clock Source
8
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265