THS4522IPW datasheet(35/71 Pages) TI2

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Rf1

1 k

500

Rg1

187

Rg2

215

Output

Measurement

Point

THS452x

Wideband,

Fully-Differential Amplifier

Rf2

1 k

50-Input Match,

Gain of 5 V/V from Rt,

Single-Ended Source to

Differential Step-Response Test

50-

Source

–

Vcc

–

Vocm

Vcc

FDA

100 nF

Rf1

1.02 k

Rload

500

Rg1

499

100 nF

Rg2

523

52.3

Output

Measurement

Point

THS452x

Wideband,

Fully-Differential Amplifier

50-Input Match,

Gain of 2 V/V from Rt,

Single-Ended Source to

Differential Output

Vocm

Rf2

1.02 k

50-

Source

–

Vcc

–

Vocm

Vcc

FDA

THS4521, THS4522, THS4524

www.ti.com

SBOS458H – DECEMBER 2008 – REVISED JUNE 2015

Device Functional Modes (continued)

Figure 74. AC-coupled, Single-ended Source to a Differential Gain of 2 V/V Test Circuit

8.4.1.2 DC-Coupled Input Signal Path Considerations for Single-Ended to Differential Conversion

The output considerations remain the same as for the ac-coupled design. Again, the input can be dc-coupled

while the output is ac-coupled. A dc-coupled input with an ac-coupled output might have some advantages to

move the input VICM down if the source is ground referenced. When the source is dc-coupled into the THS452x

family (see Figure 75 ), both sides of the input circuit must be dc-coupled to retain differential balance. Normally,

the non-signal input side has an RG element biased to whatever the source midrange is expected to be.

Providing this midscale reference gives a balanced differential swing around VOCM at the outputs.

Often, RG2 is simply grounded for dc-coupled, bipolar-input applications. This configuration gives a balanced

differential output if the source is swinging around ground. If the source swings from ground to some positive

voltage, grounding RG2 gives a unipolar output differential swing from both outputs at VOCM (when the input is at

ground) to one polarity of swing. Biasing RG2 to an expected midpoint for the input signal creates a differential

output swing around VOCM.

One significant consideration for a dc-coupled input is that VOCM sets up a common-mode bias current from the

output back through RF and RG to the source on both sides of the feedback. Without input balancing networks,

the source must sink or source this dc current. After the input signal range and biasing on the other RG element

is set, check that the voltage divider from VOCM to VIN through RF and RG (and possibly RS) establishes an input

VICM at the device input pins that is in range.

If the average source is at ground, the negative rail input stage for the THS452x family is in range for

applications using a single positive supply and a positive output VOCM setting because this dc current lifts the

average FDA input summing junctions up off of ground to a positive voltage (the average of the V+ and V– input

pin voltages on the FDA).

Figure 75. DC-Coupled, Single-Ended-to-Differential, Set for a Gain of 5 V/V

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