Overview:

The purpose of this tool is to help with the selection, evaluation and troubleshooting of voltage feedback operational amplifiers (Op-Amps). It uses typical parametric data to mathematically model the general behavior of a selected amplifier. It allows a user to select an amplifier, quickly configure a circuit, apply a signal and evaluate the general performance. It is useful for checking an amplifier's parametric performance such as Gain Bandwidth, Slew Rate, Input/Output Range, Differential Voltages, Gain Error, Load Current, Possible Stability Issues and DC Errors in a given circuit configuration. This tool is limited to first order approximations and should not be used in place of SPICE modeling or hardware testing.

Select Amplifier | Select the Circuit | Configure the Circuit | Input Signal | Analyze the Amplifier Response

How to use the tool:

Select Amplifier

Choose an Amplifier to be evaluated from the pull down, labelled "Select Amplifier," located at the top of the tool. Additional amplifiers can be added to the list by using one of the options below:

1. Parametric Search - If you know the required op-amp parameters the Parametric Search Engine can be used to find and suggest amplifiers for evaluation.   To perform a search enter the values for the desired parameters in the input blocks provided and press search.  The tool will display results that best meet the input criteria. Additional search parameters can be added by checking the desired box under "Add Searchable Parameters".  If desired, amplifiers can be selected for further evaluation by selecting the “Add Part(s) to Amplifier Parametric Evaluation Tool”, selecting the checkbox next to the desired part(s), then clicking the “Add to Tool” button at the bottom of the page.
2. Amplifier Wizard - If you're not sure of how to select an amplifier, let the Amplifier Wizard help suggest one.  The Amplifier Selection Wizard will lead you through a few generic questions and based on the response information will search for and sugest amplifiers for further evaluation. 
3. Reverse Search - Data derived from the configured circuit can be used to automatically load the parametric search engine. This allows the parametric search to look for an amplifier that will work in the circuit. If an amplifier that meets all requirements can't be found the search will suggest components that are close.

Select the Circuit

Choose the a circuit from the pull down, labelled "Select Mode," located at the top of the tool. The selections are Inverting amp, Non-Inverting amp or Difference amp configurations.

Configure the Circuit

Set the Gain

Adjusting the ratio of Rfb/Rg set the closed loop gain. Enter values for the resistors in the input blocks provided.
(Note: Large resistance values will increase errors due to input bias current and voltage noise)

Enter the Power Supply Voltages

Enter the desired Power Supply Voltage in the +Vsupply and -Vsupply input blocks provided.

Source Resistance

Enter the input signal source resistance in the block provided. The default resistance is 0 ohms. Large source resistances will increase errors due to input bias current and voltage noise.

Vref

A reference Voltage may be entered in the in the Vref block provided. The default value for Vref is 0 Volts. The Vref. voltage is used to perform level shifting of the signal.

Rbias Resistor (Inverting and Non-Inverting Modes only)

If desired enter a value for the Rbias resistor in the block provided.
(Note: In the Non-Inverting Mode Rbias resistance must be greater than 0 Ohms or the input signal will be shorted to Vref. The Rbias is used with non bias compensated op-amps to minimize errors due to input bias current. It does so by matching the source impedance seen at the non-inverting (+) pin to that seen by the inverting (-) pin. The value of Vbias is generally set to match the parallel resistance of Rfb//Rg.)

Rload

Resistive Loading of the Amplifier's Output can be set by entering a value for Rload in the block provided. The default value is 1 Gig. Ohms
(Note: Low resistance values can cause the output of the op-amp to draw excessive current)

Cload

Capacitive Loading of the Amplifier's Output can be set by entering a value for Cload in the block provided. The default value is 1 pF.
(Note: Large capacitance values can cause an op-amp to become unstable or draw high output current when used with a signal that has frequency.)

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Input Signal

Select Input Signal

Choose the Input Signal Waveform from the pull down, labelled "Input Signal," located just above the graph at the left. The available selections are Sine, Triangle and DC.

Setting the Frequency

Enter the desired Input Signal Frequency in the block provided.
(Note: This block is not shown for DC.)

Setting the Amplitude

Enter the desired Voltage Input Signal Amplitude in the block provided.
(Note: This block is not show when DC is selected, use the DC Offset block to set the Amplitude.)

Setting the DC Offset

The DC offset block is used to set the DC Offset Voltage applied to the Input Signal. In the Differential Mode this sets the average DC Voltage difference between Input Signals V2 and V1.

Setting the Common Mode Voltage (Differential Mode only)

The Common Mode Voltage block is used to set the Average DC Voltage value of Input Signals V1 and V2.

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Analyze the Amplifier Response

Running The Model

To Run the Model click on the Run Model button located at the upper right of the tool.
(Note: Each time a change is made the model will need to be manually re-run.)

Viewing Results

The calculated response for the selected amplifier is displayed in a graph on the lower right of the tool. Options for what is included in the displayed waveform are located below the graph.

Gain Error

The radio buttons provided allow the user the option of Including or Excluding gain error in the results graph. If Gain Error is excluded a caution flag will be set when the total Gain Error and DC Errors exceed 1%. If Include is selected the Gain error is included in the graph shown and in the gain block at the Right and no caution flag will be set.

Gain

The Gain indicator displays either the Ideal Gain or the Actual Gain. The Ideal Gain is defined by the values of the gain resistors. The Actual Gain is defined by the values of the Gain resistors, Feedback Factor and the Open Loop Gain of the selected amplifier at the defined input frequency.

DC Error

The radio buttons provided allow the user the option of Excluding or Including +/- DC errors in the results graph. If DC Errors are excluded a caution flag will be set when the total Gain Error and +/- DC Errors exceed 1%. If Include is selected the DC error is included in the graph shown

Output Voltage

A Digital Volt Meter (DVM) function is provided to assist in reading the Graph Voltages. The DVM functions Volts peak to peak V(P-P), Volts RMS V(RMS) and Volts DC V(DC).

Log Box

Located below the response graph at the lower right is a log box. It displays detected faults in 2 levels of error, Alerts and Cautions.

Alert

Alerts are used to flag and detail conditions under which the selected amplifier should never be operated. When an Alert flag appears in the log box no response is shown in the display graph and corrective actions must be taken before the display will show a response. Conditions that can cause an Alert include exceeding the absolute maximums for the device or possible oscillations. Suggested solutions for resolving alerts follow the Alert details.

Caution

Cautions are used to flag and detail conditions for which the selected amplifier is being operated outside it's typical performance range. When a Caution flag appears in the log box a typical first order response (derived from parametric data) is shown in the display graph. Conditions that can cause a Caution include exceeding the selected amplifier's useable input range, output range, slew rate or bandwidth. Suggested solutions for resolving cautions follow the caution details.

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