8 Detailed Description

8.1 Overview

The ultra-wide, 5.5-GHz gain bandwidth product (GBWP) of the OPA858, combined with the broadband voltage

noise of 2.5 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 OPA858 combines multiple features to optimize dynamic performance. In addition to the wide, small-signal

bandwidth, the OPA858 has 600 MHz of large-signal bandwidth (VOUT = 2 VPP), and a slew rate of 2000 V/µs.

The OPA858 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 OPA858. To reduce the effects of stray capacitance on the input node, the OPA858

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 OPA858 also features a

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

8.2 Functional Block Diagram

The OPA858 is a classic voltage-feedback operational amplifier (op amp) with two high-impedance inputs and a

low-impedance output. Standard application circuits are supported, such as the two basic options in Figure 8-1

and Figure 8-2. The dc operating point for each configuration is level-shifted by the reference voltage (VREF),

which is typically set to midsupply in single-supply operation. VREF is typically connected to ground in split-supply

applications.

+

±

VIN

RG

VREF

VREF

RF

VSIG

VREF

(1 + RF / RG) × VSIG

VOUT

VS+

VS±

Figure 8-1. Noninverting Amplifier

+

±

VIN

RG

VREF

VREF

RF

VSIG

VREF

±(RF / RG) × VSIG

VOUT

VS+

VS±

Figure 8-2. Inverting Amplifier

OPA858

SBOS629B – APRIL 2018 – REVISED MAY 2025

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