8 Detailed Description

8.1 Overview

The ultra-wide, 8-GHz gain bandwidth product (GBWP) of the OPA855-Q1, combined with the broadband

voltage noise of 0.98 nV/√Hz, produces a viable amplifier for wideband transimpedance applications, high-speed

data acquisition systems, and applications with weak signal inputs that require low-noise and high-gain front

ends. The OPA855-Q1 combines multiple features to optimize dynamic performance. In addition to the wide,

V/µs, making the device a viable option for high-speed pulsed applications.

The OPA855-Q1 is offered in a 2-mm × 2-mm, 8-pin WSON package that features a feedback (FB) pin for a

simple feedback network connection between the amplifiers output and inverting input. Excess capacitance on

an amplifiers input pin can reduce phase margin causing instability. This problem is exacerbated in the case of

very wideband amplifiers like the OPA855-Q1. To reduce the effects of stray capacitance on the input node, the

OPA855-Q1 pinout features an isolation pin (NC) between the feedback and inverting input pins that increases

the physical spacing between them thereby reducing parasitic coupling at high frequencies. The OPA855-Q1

also features a very low capacitance input stage with only 0.8-pF of total input capacitance.

8.2 Functional Block Diagram

The OPA855-Q1 is a classic voltage feedback operational amplifier (op amp) with two high-impedance inputs

and a low-impedance output. Standard application circuits are supported, like the two basic options shown

in Figure 8-1 and Figure 8-2. The resistor on the noninverting pin is used for bias current cancellation to

minimize the output offset voltage. In a noninverting configuration the additional resistors on the noninverting

pin add noise to the system so if SNR is critical, the resistor can be eliminated. In an inverting configuration

the noninverting node is typically connected to a DC voltage, so the high-frequency noise contribution from the

bias cancellation resistor can be bypassed by adding a large 1-µF capacitor in parallel to the resistor to shunt

applications.

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Figure 8-1. Noninverting Amplifier

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Figure 8-2. Inverting Amplifier

OPA855-Q1

SBOSA57A – FEBRUARY 2021 – REVISED MARCH 2021

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