9 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

9.1 Application Information

9.1.1 Wideband, High-Input Impedance DAQ Front-End

The OPA817 features a unique combination of high GBWP, low-input voltage noise, and the DC precision of

a trimmed JFET-input stage to provide a high input impedance for a voltage-feedback amplifier. Figure 9-2

shows how its very high GBWP of 400 MHz and high large signal bandwidth of 250 Mhz can be used to either

deliver wide signal bandwidths at high gains or to extend the achievable bandwidth or gain in typical high-speed,

high-input impedance data acquisition front-end applications. To achieve the full performance of the OPA817,

careful attention to the printed circuit board (PCB) layout and component selection is required as discussed in

the following sections of this data sheet. OPA817 also features a wider supply range thereby enabling a wider

common-mode input range to support higher input signal swings.

Figure 9-1 shows the noninverting gain of +2 V/V circuit used as the basis for most of the Typical Characteristics.

Most of the curves were characterized using signal sources with 50-Ω driving impedance, and with measurement

equipment presenting a 50-Ω load impedance. As Figure 9-1 shows, the 49.9-Ω shunt resistor at the VIN terminal

matches the source impedance of the test generator, while the 49.9-Ω series resistor at the VO terminal provides

a matching resistor for the measurement equipment load. Generally, data sheet voltage swing specifications are

at the output pin (VO in Figure 9-1) while output power specifications are at the matched 50-Ω load. As shown in

Figure 9-1, the total 100-Ω load at the output combined with the 250-Ω total feedback network load presents the

OPA817 with an effective output load of 83.3 Ω for the circuit.

R

F

250

–5 V

+5 V

R

G

250

0.01 μF

0.22 μF

0.01 μF

0.22 μF

49.9

V

IN

V

O

49.9

+

–

50

Source

50

Load

Figure 9-1. Noninverting G = +2 V/V Configuration

and Test Circuit

–5 V

+5 V

V

IN

V

OUT

+

–

To FDA or VGA

0.22 μF

0.01 μF

0.22 μF

0.01 μF

OPA817

50

1 M

Figure 9-2. High Input Impedance DAQ Front-End

Voltage-feedback operational amplifiers, unlike current feedback amplifiers, can use a wide range of resistor

values to set their gain. As Figure 9-1 shows, the parallel combination of RF || RG should always be kept to a

lower value to retain a controlled frequency response for the noninverting voltage amplifier. In the noninverting

configuration, the parallel combination of RF || RG will form a pole with the parasitic input capacitance at the

inverting node of the OPA817 (including layout parasitic capacitance). For best performance, this pole should be

at a frequency greater than the closed loop bandwidth for the OPA817.

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OPA817

SBOS847A – JULY 2022 – REVISED DECEMBER 2022

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Product Folder Links: OPA817