9.2 Typical Application

9.2.1 High-Input-Impedance, 180-MHz, Digitizer Front-End Amplifier

The OPA814 wide, large-signal bandwidth and high-slew rate along with high-input impedance make this device

an excellent choice for data-acquisition systems. The trimmed dc precision of the OPA814 enables the device to

be used directly as a front-end amplifier where low offset and offset voltage drift are required.

–5 V

5 V

V

IN

V

OUT

+

–

0.22 μF

0.01 μF

0.22 μF

0.01 μF

OPA814

50

900 k

100 k

R

S

100

To FDA or VGA

Figure 9-4. High-Input-Impedance, 180-MHz, Digitizer Front-End Amplifier

9.2.1.1 Design Requirements

Table 9-1 lists the design requirements for a high-input-impedance, 180-MHz, digitizer front-end amplifier.

Table 9-1. Design Requirements

PARAMETER

VALUE

Input impedance

1 MΩ or 50 Ω

Input range

1-MΩ setting

20 VPP

50-Ω setting

2 VPP

Offset drift

3.5 µV/°C, maximum

Noise at highest resolution (50-Ω Input)

90 µVRMS

9.2.1.2 Detailed Design Procedure

The following bullets list the considerations for this design example:

• Input Impedance: The JFET-input stage of the OPA814 offers gigaohms 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. Thus, the

OPA814 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 a function of the voltage and current noise of the OPA814,

input termination, and the resistors thermal noise. However, in 50-Ω mode, the dominant noise source is

contributed by the voltage noise of the OPA814 due to the presence of voltage noise across the complete

bandwidth. Therefore, the total RMS noise of the front-end amplifier is approximately equal to the voltage

noise of the OPA814 over 180 MHz.

The specified input-referred voltage noise of the OPA814 is 5.3 nV/√Hz; see also Section 7.5. The total

integrated RMS noise at the input in a bandwidth of 180 MHz is given by the following equation:

ENRMS=5.3 nV/ Hz × 180 MHz ×1.57 =90 μVRMS

(2)

The brickwall correction factor of 1.57 is applied, assuming the bandwidth is limited to 180 MHz with a

single-pole RC filter before digitizing the signal with the ADC. Detailed calculations are found at TI Precision

Labs – Op Amp Noise: Spectral Density.

OPA814

SBOSA14 – APRIL 2023

www.ti.com

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