8 Detailed Description

8.1 Overview

The ultra-wide, 1.1-GHz gain bandwidth product (GBWP) of the OPA856, combined with the broadband voltage

noise of 0.9 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 OPA856 combines multiple features to optimize dynamic performance. In addition to the wide, small-signal

The OPA856 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 OPA856. To reduce the effects of stray capacitance on the input node, the OPA856

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 OPA856 also features a very

low capacitance input stage with only 1.1-pF of total input capacitance.

8.2 Functional Block Diagram

The OPA856 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 the noise. The DC

+

±

Figure 8-1. Noninverting Amplifier

+

±

Figure 8-2. Inverting Amplifier

OPA856

SBOS623 – OCTOBER 2020

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