An acquisition system for capturing 16-bit voltage measurement data using USB

USB has become the interface of choice for connecting to PCs. Available on all modern PCs, USB interfaces provide standardized connectors that can provide 5V power to peripherals and up to 100mA of current. The circuit in Figure 1 combines a low-power 16-bit ADC, the MAX1168, from Maxim (www.maxim-ic.com), with a small USB interface module to form a simple eight-channel, 16-bit measurement system. The MAX1168 has eight input channels, an SPI (serial peripheral interface) port, a 4.096V reference voltage, and a clock oscillator. The MAX1168 operates at 5V and can convert a single channel, perform multiple conversions on a single channel, or sequentially scan multiple channels and store the measured data on-chip.

The U421 USB interface module from USBmicro (www.usbmicro.com) is based on the CY7C63743 controller from Cypress (www.cypress.com) and provides up to 16 I/O lines and an option to use some of the I/O lines as an SPI port with selectable clock rates of 62.5 kHz, 500 kHz, 1 MHz, or 2 MHz. The hardware on the U421 can access SPI read and write devices, and the device's general-purpose I/O lines can be used as slave select lines to handle a variety of SPI devices. One I/O line is used to control the chip select input of the MAX1168. When used with an HID (human interface device), the U421 USB controller can transfer data at speeds up to 800 bytes/second. The USB port adds filtering to reduce noise and can provide a 5V power supply for the circuit.

The MAX1168's sample-and-hold circuit must sample the input voltage and charge its 45 pF holding capacitor within 3ms, so a fast amplifier is needed to reduce sampling errors. Available in dual- and quad-core versions, the MAX4230 offers 10 MHz bandwidth, 2V/ms slew rate, full-swing inputs and outputs, and the ability to operate at 5V or as low as 2.7V. The MAX4230 has a typical bias current of 50 pA, which allows for fairly high input impedance without sacrificing accuracy.

To provide overvoltage protection and input voltage scaling, each buffer amplifier has a 100kΩ precision-matched resistor divider at the input. This application uses Maxim's MAX5490VA10000 10:1 divider, which provides a 1/11 scaling factor to achieve a maximum readable input of 45V with a resolution of 687.5mV. The

evaluation software uses Microsoft Visual Basic .Net Standard Edition, which provides commands to the U421 through the USBm.dll (dynamic link library) file. The demonstration program sets up the MAX1168 to scan all eight channels and displays the results. When the program is running, you can use VB (Visual Basic) to set the reference voltage for the input divider, select the scan time, and enable any of the eight input channels for screen display (Figure 2).