Concept

The measurement I/O section consists of multiple analog input and analog output channels accessed through banana plug connectors. This provides simple connectivity to the other sections of the board, both for making measurements and generating stimuli. These channels are provided by the NI ELVIS III Control I/O, and are directly routed to the banana plugs from the board edge connector.

1. Bank A analog input
2. Bank B analog input
3. Bank A analog output

The analog input section consists of two identical banks of analog inputs. Each bank includes an ADC, gain stage, mode selection, and channel multiplexer. The gain stage determines the input range for the conversion (±10 V, ±5 V, ±2 V, ±1 V). The mode selection controls whether the input is configured for single ended or differential measurement. A single ended measurement measures the difference between the selected signal and ground. A differential measurement measures the difference between the selected signal and its associated signal pair. The differential pairs are AI0/AI4, AI1/AI5, AI2/AI6, and AI3/AI7. The channel multiplexer is used to select the active channel. The two banks can operate independently from each other, or synchronously to provide simultaneous sampling on two channels.

Only channels A/AI0, A/AI2, A/AI4, A/AI6, B/AI0, and B/AI4 are exposed through the banana plugs while channels A/AI1 and A/AI5 are exposed through the fixed solderless breadboard strip. The other channels are used for other operations on the board. Refer to the Programming Details for a complete listing of NI ELVIS III channel assignments.

The analog output section consists of two identical channels of analog output. Each channel consists of a DAC and output buffer with a fixed +/-10V output range. There are no configuration settings for the analog output channels.

The voltage measurement section consists of a potentiometer driven by two voltage sources. Each source can generate arbitrary AC or DC signals, and all three terminals of the potentiometer can be measured through the banana plugs. This section can be used to demonstrate a wide variety of measurement principles, such as analog input range, resolution, and scanning vs simultaneous architectures.

Once the voltage sources are configured, you can vary the output of the middle terminal by manually turning the potentiometer. The three terminals are intended for measurement purposes only. Do not drive any voltages into these terminals. The 10 kΩ resistors provide short circuit protection, but may impact measurement quality when connected to a low impedance measurement device or circuit. For best measurement quality connect directly to a RIO analog input or oscilloscope channel.

The voltage sources are software programmable and can be independently configured to create a wide variety of circuit configurations. Some simplified representative circuits (protection resistors not shown for clarity) are as follows:

The current measurement section uses a sense resistor to measure the current flowing through a variable resistance connected to a power rail. You can turn the potentiometer to change the resistance, which will result in a change in current. By measuring the voltage across the known value of the sense resistor, Ohm's law can be used to calculate the current flow.

The nominal resistance of this circuit ranges from 464 Ω to 10.464 kΩ. The discrete resistor is included to limit the current draw from the supply when the potentiometer is turned down to its minimum value. You can programmatically select between two sense resistors, one 10 Ω the other 1 kΩ. This allows you to explore the effects of different valued sense resistors on measurement precision, noise, accuracy, and more. The two terminals are intended for measurement purposes only, and for best results a differential measurement should be used. Do not drive any voltages into these terminals. The 4.7 kΩ resistor to the banana plug protects the sense resistors in case of a short to a voltage rail. Very little current normally flows through this resistor so it does not impact the measurement results.

There are two common methods for measuring resistance. In one, a constant current source is applied to the resistor under test and the voltage is measured across the resistor. The resistance can be calculated using Ohm's law. In the other, the resistor is placed in a Wheatstone bridge and the resistance can be calculated based on the voltage measured across the center taps of the bridge. These techniques can be applied not only to actual resistors, but to resistive based sensors such as strain gauges and thermistors.

The sensor section consists of various sensor interfacing circuits to support commonly used sensors such as strain gauges, RTDs, piezoelectric sensors, phototransistors and thermocouples. You can experiment with various kinds of sensors and explore their characteristics. The output signal from a sensor can be connected to the input of other sections such as filters and amplifiers sections for further signal conditioning.

The amplification section consists of four independent amplifiers. These include a unity gain amplifier, inverting amplifier, non-inverting amplifier, and instrumentation amplifier. The inverting and non-inverting amplifiers have programmable high and low gains to optimize the output range of the amplifiers to the input range of other circuit elements. You can explore the electrical characteristics of each amplifier individually, cascade them together, or use the amplifier(s) with other elements on the board as part of a more complex signal chain. The input of an amplifier can be driven by a variety of sources, including an analog output, a function generator output, or the output of other circuit elements. The output of an amplifier can be monitored by an analog input, an oscilloscope channel, and/or be connected to the inputs of other circuit elements.

All four amplifiers share a common reference signal. In most experiments the amplifier reference should be connected to analog ground. If an experiment requires the amplifier to be referenced to an offset voltage, that voltage (instead of analog ground) can be connected to the amplifier reference.

The filtering section consists of six independent filters. These include a low pass and high pass version of a passive filter, 1^st order active filter, and 2^nd order active filter. You can explore the electrical characteristics of each filter individually, cascade a low pass and high pass filter to create a band pass filter, or use the filter(s) with other elements on the board as part of a more complex signal chain. The input of a filter can be driven by a variety of sources, including an analog output, a function generator output, or the output of other circuit elements. The output of a filter can be monitored by an analog input, an oscilloscope channel, and/or be connected to the inputs of other circuit elements.

All six filters share a common reference signal. In most experiments the filter reference should be connected to analog ground. If an experiment requires the filter to be referenced to an offset voltage, that voltage (instead of analog ground) can be connected to the filter reference.

The analog to digital conversion section consists of an instrumentation amplifier connected to a successive approximation analog to digital convertor (ADC). You can provide an analog signal into the instrumentation amplifier while monitoring the digital protocol from the ADC.

Connectivity is provided to allow a logic analyzer to monitor the digital signals, allowing for low level exploration of the protocol. The conversion sequence consists of pulsing the convert pin, waiting a fixed period of time, then performing a SPI read from the ADC. Additional probe points are provided at the output of the instrumentation amplifier, the reference for the ADC, and the mid-point of the reference which serves as the ADC input common mode voltage. Do not drive any voltages into these terminals.

This timing diagram shows a conservative use of the ADC to better illustrate the protocol and simplify the experiment setup. You do not need to use these exact timing values. However, if you choose to deviate from these values (faster or slower) you should carefully review the datasheet for the ADC and the NI ELVIS III DIO timing specifications in the NI ELVIS III Manual to ensure correct operation.

The electrical expansion section enables you to build your own circuitry for independent use or for integration with other signal chain elements built into the board. A fixed solderless breadboard strip exposes analog inputs, digital I/O, analog and digital ground, as well as connectivity to user defined banana plugs to simplify integration with the rest of the board. These channels are provided by the NI ELVIS III Control I/O, and are directly routed to the solderless breadboard from the board edge connector. A large, removable solderless breadboard is available for your custom circuits.

1. Fixed solderless breadboard with I/O
2. User banana plug connectivity
3. Removable solderless breadboard

There are two analog input channels, A/AI1 and A/AI5. They can be configured as a single differential pair or two single ended inputs, and for input ranges of ±10 V, ±5 V, ±2 V, or ±1 V. They share the same analog input subsystem as the bank A analog input described in the measurement I/O section. Refer to the Programming Details for a complete listing of NI ELVIS III channel assignments.

There are eight digital input/output channels, A/DIO0:7. They can be individually configured as inputs or outputs. They can be used as static digital lines, pattern generation outputs, counter inputs, and more. They are 5 V tolerant, 3.3 V output, LVTTL compatible lines.

There is a connection for analog and digital ground. Ensure the correct type of ground is used for return currents, otherwise there may be additional noise in the signal paths.

There are four user banana plugs electrically connected to the fixed solderless breadboard. These can be used to connect circuitry implemented on the removable solderless breadboard with the other circuit elements on the board.

The mechanical expansion section enables you to replace the removable solderless breadboard with your own boards or devices. These could be custom circuit boards implementing an alternative signal chain, sensors mounted to representative structures such as a strain gauge on a flexible beam, or an entire mini-plant representing an overall process.

The removable solderless breadboard is attached to the PCB by seven small screws accessible from the bottom side of the board. Note that the mounting holes for the solderless breadboards are not designed for repeated attachment and removal, and doing so may result in damage to the plastic threads. Once removed, there are 15 holes in the board that can be used for mechanical mounting purposes. There are the seven smaller holes that hold the removable solderless breadboard, and eight additional larger holes that are available for more secure mounting of devices.

LEDs on the board illuminate to indicate which circuit elements are used by a given experiment, simplifying experiment setup. Some experiments will require that multiple LEDs be turned on (for example, turning on the LEDs for the quarter bridge, the amplifier section, and the filter section). The 15 LEDs are controlled by 9 digital outputs from the ELVIS III, using two 3-to-8 decoders and three direct digital lines. This reduces the number of digital outputs required to enable the LEDs while still allowing the valid multiple LED combinations to be displayed.

These digital outputs can be programmed from LabVIEW FPGA or LabVIEW RT.

The two voltage sources are provided by analog outputs from the NI ELVIS III. These connections are built into the NI Automated Measurements Board; the student does not have to perform any wiring. These analog outputs are shared with multiple sections on the board. Each section has a dedicated enable signal controlled by a digital output from the NI ELVIS III.

These digital output and analog outputs can be programmed from LabVIEW FPGA or LabVIEW RT.

The current measurement section includes two sense resistors. The active sense resistor is selected by a digital output from the NI ELVIS III.

These digital outputs can be programmed from LabVIEW FPGA or LabVIEW RT.

The inverting and non-inverting amplifiers have a programmable gain setting. The gain settings are selected by digital outputs from the NI ELVIS III.

These digital outputs can be programmed from LabVIEW FPGA or LabVIEW RT.

The Quarter Bridge and Full Bridge sections share a common +5 V excitation source. More accurate bridge measurements can be made by knowing the precise value of the excitation source. A differential analog input is connected to the excitation source and its reference. This connection is built into the NI Automated Measurements Board; the student does not have to perform any wiring.

This section uses the same +5 V excitation source as the Quarter Bridge and Full Bridge sections, however here it is referenced directly to analog ground. The same analog input can be used to more accurately measure the excitation voltage, only it should be configured for a single ended measurement for this section (B/AI1). This connection is built into the NI Automated Measurements Board; the student does not have to perform any wiring.

The thermocouple screw terminals are connected directly to an NI ELVIS III differential analog input in order to reduce noise and minimize additional cold junction connections. This connection is built into the NI Automated Measurements Board; the student does not have to perform any wiring. The positive terminal of the thermocouple is connected to B/AI2 and the negative terminal to B/AI6.

To support Cold Junction Compensation for the thermocouple, retrieve the board temperature from the on-board temperature sensor through its I²C device address of 0x4F. The digital lines used are A/DIO14 and A/DIO15 for SCL and SDA respectively.