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 OPA814 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 the very high GBWP of 250 MHz and high large signal bandwidth of 200 MHz are 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 OPA814,

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

the following sections of this data sheet. The OPA814 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 a +2

‑V/V circuit used as the basis for most of the Typical

Characteristics. Most of the curves are 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); whereas, output power specifications are at

the matched 50-Ω load. Figure 9-1 shows that the total 100-Ω load at the output combined with the 500-Ω total

feedback network load presents the OPA814 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

OPA814

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

OPA814

50

1 M

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

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

to set the gain. As Figure 9-1 shows, the parallel combination of RF || RG must 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 form a pole with the parasitic input capacitance at the inverting node of the

OPA814 (including layout parasitic capacitance). For best performance, this pole must be at a frequency greater

than the closed-loop bandwidth for the OPA814.

OPA814

SBOSA14 – APRIL 2023

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