Texas Instruments OPA455EVM Manuale utente

User’s Guide

OPA455EVM Evaluation Module

ABSTRACT

This user's guide describes the characteristics, operation, and use of the evaluation module (EVM) for the

OPA455. The EVM is designed to evaluate the performance of the devices in both single and dual-supply

configurations. This document also includes the schematic, printed circuit board (PCB) layout, and a bill of

materials (BOM). Throughout this document the terms evaluation board, evaluation module, and EVM are

synonymous with OPA455EVM.

WARNING

Danger: HIGH VOLTAGE! This evaluation board is intended for professional use only. It has exposed

high voltages. Do not operate this board without proper high-voltage/high-current safety practices.

Read Section 1.4 before using the EVM.

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OPA455EVM Evaluation Module 1

Table of Contents

1 Overview..................................................................................................................................................................................4

1.1 OPA455..............................................................................................................................................................................4

1.2 OPA455EVM...................................................................................................................................................................... 4

1.3 Related Documentation......................................................................................................................................................4

1.4 Evaluation Module Limitations and Cautions..................................................................................................................... 4

2 Getting Started........................................................................................................................................................................5

2.1 Power Supplies.................................................................................................................................................................. 5

2.2 Jumpers..............................................................................................................................................................................5

2.3 Inputs................................................................................................................................................................................. 5

2.4 Outputs...............................................................................................................................................................................5

2.5 Enable or Disable (E/D)..................................................................................................................................................... 6

2.6 Status Flag......................................................................................................................................................................... 6

3 Application Circuits................................................................................................................................................................7

3.1 Improved Howland Current Pump......................................................................................................................................7

4 Schematic, PCB Layout, and Bill of Materials................................................................................................................... 10

4.1 EVM Schematic................................................................................................................................................................10

4.2 EVM Default Configuration...............................................................................................................................................11

4.3 PCB Layout...................................................................................................................................................................... 12

4.4 Bill of Materials.................................................................................................................................................................13

List of Figures

Figure 3-1. Practical, 20-mA, Improved Howland Current-Pump Circuit..................................................................................... 7

Figure 3-2. Improved Howland Current Pump Output-Voltage Compliance Range.................................................................... 8

Figure 3-3. Improved Howland Current Pump Applied as a Peak AC-Current Generator...........................................................9

Figure 4-1. OPA455EVM Schematic..........................................................................................................................................10

Figure 4-2. OPA455EVM Default Configuration.........................................................................................................................11

Figure 4-3. OPA455EVM PCB Layout....................................................................................................................................... 12

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List of Tables

Table 1-1. Related Documentation.............................................................................................................................................. 4

Table 2-1. Jumper Table.............................................................................................................................................................. 5

Table 4-1. OPA455EVM BOM....................................................................................................................................................13

Trademarks

PowerPAD™ is a trademark of Texas Instruments.

All other trademarks are the property of their respective owners.

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OPA455EVM Evaluation Module 3

1 Overview

1.1 OPA455

The OPA455 is a precision power amplifier capable of operating over a very wide supply range of ±6 V to ±75 V,

with an input common-mode range that extends to within a few volts from each rail. This amplifier is composed

of an internal differential input amplifier stage, followed by a voltage amplifier, and finally, a high-current output

stage. This device is designed to avoid phase inversion problems that are typically found in similar op amps. The

OPA455 is unity-gain stable, and comes in an SO PowerPAD™ package.

1.2 OPA455EVM

The OPA455EVM is intended to provide basic functional evaluation of the OPA455. The EVM provides the

following features:

• Easy access to pertinent nodes with test points and terminal blocks

• Convenient input and output filtering

• Access to multiple nodes and footprints for compensation

• Protection against power supply sequencing and inductive load kickback

• Configurable for any common amplifier topology, including the improved Howland current pump

• Footprints for providing a flexible reference voltage

1.3 Related Documentation

The documents listed in Table 1-1 provide information about TI's integrated circuits and support tools for the

OPA455EVM.

Table 1-1. Related Documentation

Document Literature Number

OPA455 Product Data Sheet SBOSA16

OPA462 Product Data Sheet SBOS803

OPA445 Product Data Sheet SBOS156

1.4 Evaluation Module Limitations and Cautions

The OPA455EVM is designed to safely withstand the maximum rated supply, input and output voltages, and

current levels specified for the OPA455 high-voltage op amp. In addition to observing maximum voltage and

current limitations, proper electrostatic discharge precautions are advised when handling and applying the EVM.

1.4.1 High-Voltage Warning

The OPA455 is rated for supply voltages up to ±75 V (150 V for single supply), and common-mode input

voltages up to ±70 V. When using the device and EVM under such circumstances, all proper safety practices

must be followed. Do not apply more than rated voltages. See the OPA455 data sheet for additional information.

1.4.2 Electrostatic Discharge Caution

Many of the components on the OPA455EVM are susceptible to damage by electrostatic discharge (ESD).

Customers are advised to observe proper ESD handling precautions when unpacking and handling the EVM,

including the use of a grounded wrist strap at an approved ESD workstation.

CAUTION

A wrist strap may only be used with circuit voltages less than 100 V. For circuit voltages equal to or

greater than 100 V, use an ionizer.

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2 Getting Started

This section explains the connectors and jumpers, and details the basic EVM functionality.

2.1 Power Supplies

The OPA455EVM can be configured from ±6 V to ±75 V for dual-supply operation, or from 12 V to 150 V for

single-supply operation. Shorting pins 2 and 3 of J5 sets the EVM to single supply, and shorting pins 1 and 2 of

J5 sets up the EVM for dual supplies. The OPA455 is capable of sourcing or sinking up to 30 mA of current.

Power is provided to the EVM through J2 (V+) and J1 (V–). Take care to select supplies capable of providing the

required power for the application. Use a supply capable of supplying at least twice the anticipated current at the

intended operating voltage.

2.2 Jumpers

There are six jumpers located on the OPA455EVM. The jumper functions are summarized in Table 2-1.

Table 2-1. Jumper Table

Jumper Function

J3 Enable or disable (E/D) common configuration

J4 Enable or disable (internal or external)

J5 Sets the negative supply to GND for single-supply operation.

J7 Selects inverting input (external or GND)

J10 Selects noninverting input (external or GND)

J12 Sets the reference voltage at the noninverting pin

2.3 Inputs

The inputs to the EVM can be any signal within the common-mode voltage range. BNC connectors are available

at both inputs to allow for easy interfacing with signal generators or other equipment. Additionally, signals can be

input using test points TP12 (IN–) and TP14 (IN+).

Jumpers J7 and J10 set the voltages seen at IN– and IN+, respectively. Short pins 2 and 3 of these jumpers to

set the inputs to GND. Short pins 1 and 2 to allow whatever voltages present at the input BNCs to be passed to

the op amp.

The OPA455 non-inverting input may be set up to have a voltage divider in its path. PC board resistor locations

R8 and R12 would need to be populated with the appropriate divider resistances. A jumper is then added

between J12, pins 2 and 3, to complete the divider. Additionally, J12 may be used to route an externally-applied

reference voltage from the TP15 (VREF Ext) test point, to the OPA455 non-inverting input. A jumper is placed

across J12, pins 1 and 2, to establish the connection.

2.4 Outputs

The output of the EVM can be monitored from TP13. A BNC connector, J8, and terminal block, J11, are also

provided to interface with other equipment or circuits. The EVM includes footprints for a load resistor, load

capacitor, and an RC snubber network. If significant power is going to be driven into the load, use external loads.

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2.5 Enable or Disable (E/D)

The OPA455 incorporates a disable feature that can be set using jumpers J3 and J4 on the EVM. The disable is

accomplished without disturbing the input signal path, which helps save power in addition to protecting the load.

Jumper J3 sets the voltage of the E/D Com pin, which can be set to either the negative rail or GND. The voltage

at the E/D Com pin serves as the reference voltage for the E/D pin. J4 sets the voltage at the E/D pin. The

device is enabled by floating the E/D pin, or by forcing the E/D pin high using an external voltage. If forcing the

E/D pin high, the pin must be pulled at least 2.5 V greater than the E/D Com pin, but should not be pulled any

more than 5 V greater than the E/D Com pin. Shorting the E/D pin to the E/D Com pin disables the device.

2.6 Status Flag

The OPA455 incorporates a status flag pin to indicate overcurrent or overtemperature conditions. This pin is an

open-drain, active-low output that allows the pin to be easily interfaced with standard low-voltage logic. The pin

goes active if the junction temperature of the device rises to greater than 150°C, and resets after the device

cools to 130°C. Additionally, the pin goes active if the device attempts to source more than the maximum

continuous current of 30 mA. The status flag can be monitored from TP2, and an external pullup voltage can be

input on TP11.

2.6.1 Circuit Protection

The OPA455EVM is protected from inductive surges and power supply transients by ultra-fast rectifier diodes at

the amplifier output pin, and transient voltage suppressors (TVS) at the power supply inputs. See D1, D2, D3,

and D4 on the schematic shown in Figure 4-1, and in the BOM shown in Table 4-1.

In addition, footprints have been provided for optional components Q1 and Q2. These may be used to protect

the amplifier inputs from voltage transients that briefly exceed the maximum input voltage. See the BOM at the

end of this document for the recommended part number.

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3 Application Circuits

The OPA455EVM may be configured in most standard op amp circuits. See the OPA455 data sheet for typical

applications.

3.1 Improved Howland Current Pump

The OPA455EVM can be configured as an improved Howland current pump that provides an output current

proportional to a single- or differential-input voltage. The improved Howland current pump is described and

analyzed in section 3 of A Comprehensive Study of the Howland Current Pump Application Report. Information

about how the current-pump resistor values are determined for a particular input voltage and a corresponding

output current is provided in the application report. The report information is applied here in this section as a

practical use for the OPA455EVM.

The OPA455EVM, as configured in Figure 3-1, sources on an output current of –20 mA when a single-ended

input voltage of 2.5 V is applied to the VP input. If the input voltage sign is reversed, the output reverses

direction and sinks 20 mA. The circuit in Figure 3-1 provides the resistance values required to obtain the correct

20-mA output current. The resistor and capacitor labels correspond to those screened on the OPA455EVM

printed circuit board. As shown in Figure 3-1, when RL is equal to 500 Ω, the sourced output current level is –20

mA, and Vout is 10 V, as Ohm’s law dictates.

Figure 3-1. Practical, 20-mA, Improved Howland Current-Pump Circuit

Practical current sources have a limited output voltage range that can be provided. The OPA455 op amp has a

wide output voltage range because of the high-voltage supplies used. The OPA455 data sheet provides

information about the linear output voltage range that should always be observed.

For the improved Howland current pump application, the output voltage (Vout) that is attainable at the load is

further constrained by the voltage drop developed across R13. The 20-mA load current flows through the

resistor, and develops a voltage drop. Therefore, with a fixed 20-mA output current flowing through RL, there is a

limit as to how far Vout can swing. For this example, the limit occurs when Vout reaches approximately 70.5 V,

and RL is 3.53 kΩ. This limit can be seen in Figure 3-2. Resistor RL can be any value from 0 Ω to 3.53 kΩ, and

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OPA455EVM Evaluation Module 7

the output will remain within the circuit linear compliance range. However, if the value of RL is increased further,

the current pump operation is forced outside the compliance range, and the output current will no longer remain

constant. Thus, increasing the value of RL to greater than 3.53 kΩ defeats the goal of maintaining a constant

current.

Figure 3-2. Improved Howland Current Pump Output-Voltage Compliance Range

The improved Howland current pump can be used to generate an ac waveform with a peak current that

corresponds to a maximum specified DC value. A 20-mA DC current source using the OPA455 has been

demonstrated, and will now be used to show how a 20-mA peak AC-current waveform can be realized.

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The same circuit shown in Figure 3-1 is applied, except the 2.5-VDC VP voltage source in Figure 3-1 is replaced

by an 100-Hz AC sine wave source having a peak voltage of 2.5 V. The peak output current depends on the

peak voltage level applied at the VP input. A sine wave is used in the example; however, a triangle wave, square

wave, or other waveform could be applied instead. Figure 3-3 shows that a 2.5-V peak input sine wave applied

to the current pump input, VP, results in a 20-mA peak sine wave at the output, Vout.

Figure 3-3. Improved Howland Current Pump Applied as a Peak AC-Current Generator

The improved Howland current pump op amp circuit relies on both negative and positive feedback for operation;

more negative feedback than positive feedback, but that feedback does not always provide stability. When unity-

gain-stable op amps, such as the OPA455, are employed and they drive a resistive load, the op amp phase

margin should be sufficient such that the circuit is stable. However, if the output load is complex, containing both

resistive and reactive components (R±jX), there can be combinations that degrade the phase margin, and

instability results. Degradation of the phase margin often occurs when the current pump is used to drive loads

that are predominantly inductive.

The OPA455EVM has a number of positions where component may be added to stabilize the various OPA455

amplifier and improved Howland current pump configurations that are supported by the EVM. Some of the

component locations that are used for this purpose are R6 (ISO), R5 (Rng), C12 (Cng, noise gain), R10 (Rsn),

C16 (Csn, snubber), and C9 and C10 in the feedback loop. Compensation, when required, is determined based

on the particular circuit to which the OPA455 is being applied. Op amp stability and compensation is a vast

subject covered in numerous TI documents, and TI training programs such as TI Precision Labs – Op Amps.

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4 Schematic, PCB Layout, and Bill of Materials

This section provides the OPA455EVM hardware schematics and PCB layout.

4.1 EVM Schematic

Figure 4-1 depicts the complete OPA455EVM schematic.

Figure 4-1. OPA455EVM Schematic

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