Texas Instruments OPA455EVM User manual
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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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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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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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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