Input-Output Head (A2057) Manual

Contents

Description

The Input-Output Head (A2057) is a LWDAQ device that provides two general-purpose analog inputs, two analog outputs with eight-bit resolution, and four open-drain logic outputs. All version of the A2057 allow us to read voltages using the LWDAQ Voltmeter Instrument.

Figure: The Input-Output Head (A2057A) with A205701F circuit board.

The Input-Output Head (A2057A) uses the A205701F circuit board. It provides three 4-way plugs for the inputs and outputs, as well as a grid of holes for additional circuits and connectors.

Figure: The Voltmeter (A2057V) on the A205701G printed circuit board.

The Voltmeter Head (A2057V) uses the A205701G circuit board and houses it in a PPL enclosure by PAC-TEC. The A2057V provides two BNC plugs for the two analog inputs, allowing them to be used with oscilloscope probes. The LWDAQ device socket is set in the end wall opposite the BNC connectors. Two 4-way plugs provide four digital outputs and two analog outputs. The enclosure as shown does not give external acess to the digital or analog outputs. It is easy to cut additional holes in the cover.

Figure: Voltmeter (A2057V) with Enclosure.

The Wheatstone Bridge Head (A2057W) adds a differential amplifier to the front end of the A2057A, as well as a 10-V power supply, as shown here. The A2057W allows us to power and read out most Wheatstone Bridge sensors. We connect the sensor to the new J100 plug, which provides 10-V power and accepts a differential input A/B from the bridge.

Figure: The Wheatstone Bridge Head (A2057W) with A205701F circuit board.

In the compact A2057W implementation shown above, we cut traces to J4 and put J100 in its place. All differential amplifier parts are installed on the patch area provided by the circuit board. Note that the A205701F circuit board does not leave adequate clearance for metal standoffs in its top-left corner. There are several vias that will make contact with a hex nut on the top side of the board.

Connectors

All plugs on the A2057 are mis-named as sockets on the printed circuit boards and in the schematics. They should be identified as Pn instead of Jn. The four-way shrouded plugs are 0.1" pitch headers, part number 70543-0003 from Molex-Waldom. To mate with these connectors, we use crimp terminal 16-02-0096 and four-way crimp housing 50-57-9004. Pin one (1) on each plug is on the left when looking from the nearest board edge, and is identified also by a square pad.

The A2057A and A2057V use the A205701F and A205701G printed circuit boards respectively. These circuit boards differ in layout and in the presentation of the analog inputs. The A2057A provides two analog inputs on a four-way plug with the following pin assignments.

The A2057V replaces connector J2 with two BNC plugs named J5 and J6. These provide connection to X1 and X2 respectively.

The A2057A and A2057V provide the same pinout for J3 and J4.

The A2057W is built on the A205701F circuit board. We cut the traces to the J4 connector and use the J4 footprint for the four-way J100 connector.

The Wheatstone Bridge amplifier circuit has inputs A and B and produces an output 101×(A−B). This output is called XX1 and connects to J2-1, where it is buffered and becomes X1, the first analog input of the A2057. Another output from the amplifier is A itself, which connects to J2-3, and so becomes X2.

Command Bits

The table below gives the assignment of LWDAQ command bits to the A2057 functions.

To determine the command word that will implement a particular operation on the A2057, write out sixteen bits in a row, starting with bit sixteen (DC16) on the left, and ending with bit one (DC1) on the right. Set each bit to zero or one as you require. The left-most four bits form the most significant nibble of the sixteen-bit command word. The right-most four bits are the least significant nibble. Translate each nibble into a hex digit, and you have the hex version of the command word.

The A2057 provides four analog input channels which are internally multiplexed together, passed through several op-amp gain stages, and sent to the driver as a low voltage differential signal for 16-bit analog to digital conversion. The user may use two of the input channels (ON1, ON2) to acquire the desired voltage source. The two remaining input channels, ON3 and ON4, are connected to 0V and 5V respectively. These are used as reference channels for the A2057's self-calibration. The user may choose one of two gains to apply to the input signals, ×1 and ×10, using the GSEL command bit.

The two input channels, ON1 and ON2, can accept signals of ±15V, and no greater than 10kHz, limited by 16-bit ADC and 10kHz low pass filter on the driver.

The A2057 uses two Texas Instrument's TLV5623 8-bit digital-to-analog converters. The output of each DAC is passed through an adjustable op-amp gain stage.

The A2057 uses four NDS355AN n-channel mosfets. When OUTn is asserted, signal OUT1 will be open-circuit. When OUTn is unasserted, OUT1 will be connected to 0V by a 0.1Ω resistance. For OUT1 to be asserted, we must set the !OUT1 bit (also called DC1) to zero.

Operation

We control and read out the A2057 with the Voltmeter Instrument in our LWDAQ Software. The Voltmeter allows us to read the analog inputs and set the digital outputs. But the current version of the instrument does not allow us to set the analog outputs. You can, however, set the analog outputs with a TclTk script, as we describe below. Furthermore, the A2057 Tester script is a LWDAQ Tool that allows us to test all aspects of the A2057. You can run it with "Run Tool" in the Tool menu.

You will find the data acquisition steps required to control and read out the A2057 in Voltmeter.tcl, which is the TclTk script that defines the Voltmeter Instrument. In Driver.tcl you will find the routines that compose TCPIP messages to communicate with a LWDAQ Driver.

Analog Inputs

Here are the data acquisition steps required to read out the A2057 inputs.

1. Plug one end of a CAT-5 cable into the A2057 connector J1, and the other end into a driver socket. Take note of selected driver socket.
2. Connect a signal source to the A2057. For A205701F: Plug a cable into the J2 Molex connector and attach the other end to a signal source. For A205701G: Plug a x10 scope probe into one or both of the J5, J6 BNC connectors, and attach the other end to a signal source. Take note of which of the selected connector
3. Tell the driver which driver socket is being used.
4. Compose a command word which asserts some variation of the following bits: WAKE, LB, ON1, ON2, ON3, ON4, GSEL. The WAKE and LB bits must be asserted for operation. Bits ON1, ON2, ON3, ON4 are channel specific, and of these four only one may be asserted, the rest must be unasserted. The GSEL bit left unasserted corresponds to x1 gain, while GSEL asserted corresponds to x10 gain. For example the command word for input channel 1, gain ×1 is as follows: 0000000011010000.
5. To utilize the A2057's ability to self-calibrate, the user may select voltage reference channels ON3 and ON4. ON3 selects the 0V reference channel, ON4 selects the 5V reference channel.
6. Execute the following LWDAQ driver device jobs:
1. Set the RAM data address to 0.
2. Execute a command job with the command word composed earlier.
3. Repeat step 2 for each reference channel.
4. Set repeat counter.
5. Set delay timer.
6. Execute adc16_job. Data is now stored in ram.
7. Read data from RAM.

You will find the above steps implemented with Tcl in Voltmeter.tcl.

Analog Outputs

The outputs of the two DACs are located on connector J3 as described above. The Voltemeter Instrument allows us to read the A2057's analog inputs and set its digital outputs. But it does not allow us to set the analog outputs. To set the analog outputs, we must send a sequence of commands to the A2057's 8-bit DACs. These commands control the DAC serial data input and serial clock directly, and so clock in a new sixteen-bit DAC word, of which eight bits dictate the analog output from the TLV5623 eight-bit DAC. Do not confuse the 16-bit LWDAQ command with the 16-bit DAC control word. We use a sequence of LWDAQ commands to send a single DAC control word.

The first four bits of the DAC's sixteen-bit control word are control bits. The next eight bits are the DAC value. The final four bits are all zeroes. The A2057 uses six command bits to control the DACs. These bits are DIN (DC16), SCLK (DC15), FS (DC14), DAC2 (DC13), DAC1 (DC11), and WAKE (DC8). The DAC1 and DAC2 bits select which DAC to use. Only one should be asserted at a time. The WAKE bit must be asserted to provide power to the A2057. The FS bit is called Frame Sync, and must be asserted once at the beginning of each data word transfer, and once again at the end of each data word transfer. The DIN bit is the serial data bit for the DACs. The rising edge of SCLK clocks DIN into the selected DAC.

A sixteen-bit control transmission to a DAC requires a total of 35 LWDAA command transmissions, which takes 140 μs. The maximum square-wave output frequency of the DACs is 3.5 kHz. The list below gives an example command word sequence that sends one control word to DAC1. The DAC integer value is 6, or binary 00000110. We give the LWDAQ commands as hex values. Note that the most significant nibble pulses SCLK, and the last three nibbles stay the same for a given DAC.

The DACs' outputs feed into their own adjustable gain op-amp stage. The A2057 is built with x1 gain. The user may adjust the gain by choosing values of resistors which provide the appropriate gain according to the equations found on Page 6 of the schematic.

Electronics

Note: All our schematics and Gerber files are distributed under the GNU General Public License.

Schematic

PCBs