9.2 Typical Applications

9.2.1 High Input Impedance, 200 MHz, Digitizer Front-End Amplifier Design

The OPA817 offers a wide large-signal bandwidth, high-slew rate along with high-input impedance making it

ideal for data acquisition systems. The trimmed DC precision of the OPA817 enables its use directly as front-end

amplifier where low offset and offset voltage drift is needed.

–5 V

+5 V

V

IN

V

OUT

+

–

0.22 μF

0.01 μF

0.22 μF

0.01 μF

OPA817

50

900 k

100 k

R

S

100

To FDA or VGA

Figure 9-3. High Input Impedance, 200 MHz, Digitizer Front-End Amplifier

9.2.2 Design Requirements

Table 9-1 lists the design requirements for a High Input Impedance, 200 MHz, Digitizer Front-End Amplifier.

Table 9-1. Design Requirements

Specification

Value

Input Impedance

1 MΩ / 50Ω

Input Range (1 MΩ / 50 Ω)

20 VPP / 2 VPP

Offset Drift

3.5 µV/°C maximum

Noise at Highest Resolution (50 Ω Input)

80 µVRMS

9.2.3 Detailed Design Procedure

• Input Impedance: The JFET-input stage of the OPA817 offers giga ohm's of input impedance and therefore

enables the front-end to be terminated with a 1 MΩ resistor while achieving excellent precision. A 50 Ω

resistance can also be switched in offering matched termination for high-frequency signals. The OPA817

therefore enables the designer to use both 1 MΩ and 50 Ω termination in the same signal chain.

• Noise: The total noise of the front-end amplifier is the function of the voltage and current noise of the

OPA817, input termination, and the resistors thermal noise. In 50 Ω mode, the dominant noise source,

however, is contributed by the voltage noise of the OPA817 due to its presence across the complete

bandwidth. Thus, the total RMS noise of the front-end amplifier shall be approximately equal to the voltage

noise of OPA817 over 200 MHz.

The specified input referred voltage noise of the OPA817 is 4.5 nV/√Hz; for more information see Section 7.5.

The total integrated RMS noise at the input in a bandwidth of 200 MHz is given by the following equation:

EnRMS = 4.5 nV/√Hz × √ (200 MHz × 1.57) = 80 µVRMS.

(1)

The Brickwall correction factor of 1.57 is applied assuming the bandwidth will be limited to 200 MHz with

a single pole RC-filter before digitizing the signal with the ADC. Detailed calculations can be found on TI

Precision Labs – Op Amps: Noise – Spectral Density.

OPA817

SBOS847A – JULY 2022 – REVISED DECEMBER 2022

www.ti.com

18

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