External

Pin

+V

CC

Internal

Circuitry

OPA657

www.ti.com

SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015

8 Detailed Description

8.1 Overview

The OPA657 is high gain-bandwidth, voltage feedback operational amplifier featuring a low noise JFET input

stage. The OPA657 has been decompensated to allow for optimized Bandwidth to Quiescent current

performance and better optimize its voltage noise performance. The OPA657 finds wide use in optical front-end

applications and in test and measurement systems that require high input impedance. It is built using a very high

speed complementary bipolar process.

8.2 Feature Description

8.2.1 Input and ESD Protection

The internal junction breakdown voltages are relatively low for these very small geometry devices. These

breakdowns are reflected in the Absolute Maximum Ratings table. All device pins are protected with internal ESD

protection diodes to the power supplies, as shown in Figure 28.

These diodes provide moderate protection to input overdrive voltages above the supplies as well. The protection

diodes can typically support 30-mA continuous current. Where higher currents are possible (that is, in systems

with ±12-V supply parts driving into the OPA657), current limiting series resistors should be added into the two

inputs. Keep these resistor values as low as possible because high values degrade both noise performance and

frequency response.

Figure 28. Internal ESD Protection

8.3 Device Functional Modes

8.3.1 Split-Supply Operation (±4-V to ±6-V)

To facilitate testing with common lab equipment, the OPA657 may be configured to allow for split-supply

operation. This configuration eases lab testing because the mid-point between the power rails is ground, and

most signal generators, network analyzers, oscilloscopes, spectrum analyzers and other lab equipment reference

their inputs and outputs to ground. Figure 29 and Figure 30 show the OPA657 configured in a simple

noninverting and inverting configuration respectively with ±5-V supplies. The input and output will swing

symmetrically around ground. Due to its ease of use, split-supply operation is preferred in systems where signals

swing around ground, but it requires generation of two supply rails.

8.3.2 Single-Supply Operation (8-V to 12-V)

Many newer systems use single power supply to improve efficiency and reduce the cost of the extra power

supply. The OPA657 is designed for use with split-supply configuration; however, it can be used with a single-

supply with no change in performance, as long as the input and output are biased within the linear operation of

the device. To change the circuit from split supply to single supply, level shift all the voltages by 1/2 the

difference between the power supply rails. An additional advantage of configuring an amplifier for single-supply

operation is that the effects of -PSRR will be minimized because the low supply rail has been grounded.

Copyright © 2001–2015, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: OPA657