THS3215 Datasheet, PDF (60/71 Pages) Texas Instruments – THS3215 650-MHz, Differential to Single-Ended DAC Output Amplifier

THS3215
SBOS780A – MARCH 2016 – REVISED APRIL 2016
10.1.1.5 Differential I/O Driver With independent Common-Mode Control
VMID_OUT
VO1
16
15
+
VMID_IN
±
1
50 k
2 x1
50 k
x1
50
49.9
Differential
VIN
+
250
500
3 x1
49.9
14
100
13
18.5 k
294
12
294
11
VOUT = VMID_OUT ± 2VIN
Differential
Filter
10
18.5 k
Differential
Input Mixer
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VPATHSEL > 1.3 V
4
9
VMID_OUT
5
6
7
8
VO1= VMID_OUT + 2VIN
Figure 105. Differential I/O with Common-Mode Control
10.1.1.5.1 Detailed Design Procedure
Certain applications require the differential DAC output voltage to be translated from one common-mode
(compliance) level to a differential output at a different common-mode level. The THS3215 performs voltage-level
translation directly using the very flexible blocks provided internally. Figure 105 shows an example of such an
application, where the differential gain is always 4 V/V. The differential gain is fine-tuned down by setting the
insertion loss in the differential post-filter. The considerations critical to this application include:
1. The input is dc-coupled with the appropriate termination impedance required by the DAC. Use a high-
frequency, antialiasing filter at the input to limit DAC feedthrough in the deselected OPS internal input.
2. The output common-mode control is set with the voltage applied to the VMID buffer input at VMID_IN (pin 1).
The circuit is configured so that the output at VMID_OUT (pin 15) drives both VREF (pin 14), in order to set
the D2S dc output voltage, and VIN+ (pin 9).
3. The D2S output available at VO1 (pin 6) provides one side of the differential-output, and is dc-biased at
V_{MID_OUT}. VO1 also drives the R_G resistor for the OPS in an inverting gain of –1 V/V. The dc bias level at the
R_G input and the VIN+ input of the OPS are the same voltage; therefore, no level shift through the OPS
occurs. The OPS outputs an inverted version of the D2S output signal at the same common-mode voltage
(V_{MID_OUT}). The wideband, differential signal with independent output common-mode voltage control can now
be applied to a differential filter and on to the next stage.
4. Make sure that the differential filter has only differential resistors and capacitors. Termination resistors to
ground level-shift the input common-mode voltage, while differential resistors transfer V_{MID_OUT} directly
through the filter as a common-mode input to the mixer.
5. If the desired V_{MID_OUT} + differential signal combined clips in the OPS or D2S, offset the supplies to gain
headroom. For instance, if a 5-V output common-mode voltage is required with a 10-V_PP differential signal,
the OPS and D2S must deliver 2.5-V to 7.5-V output swings. The D2S has the higher headroom requirement
at 1.5 V (max). Operating the THS3215 with –5 V and 10 V supplies stays within the rated maximum of 15.8
V total supply range, and provide adequate headroom for the positive offset swing requirement. Note that the
logic lines are still referenced to GND by pin 7. Tie PATHSEL (pin 4) to +V_CC to hold this design in the
external path mode required.
60
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