Data-acquisition system captures 16-bit voltage measurements using the USB

The USB has become the interface of choice for connecting to PCs. Available on all relatively modern PCs, the USB offers a standard connector and can supply power to peripherals at 5V and as much as 100 mA of current. The circuit in Figure 1 combines Maxim's (www.maxim-ic.com) MAX1168, a low-power, 16-bit ADC, with a small USB-interface module to make a simple, eight-channel, 16-bit measurement system. The MAX1168 includes eight input channels, an SPI (serial-peripheral-interface) port, a 4.096V reference, and a clock oscillator. The MAX1168 operates from a 5V supply and can convert individual channels, execute multiple conversions on one channel, or scan the channels sequentially and store measured data on-chip.

Based on a Cypress (www.cypress.com) CY7C63743 controller, USBmicro's (www.usbmicro.com) U421 USB-interface module provides as many as 16 I/O lines and an option to use some of those lines as an SPI port at selectable clock rates of 62.5 kHz, 500 kHz, 1 MHz, or 2 MHz. Firmware on the U421 allows generic access to SPI read-and-write devices, and the device's general-purpose I/O lines can serve as slave-select lines for addressing multiple SPI devices. One I/O line controls the MAX1168's chip-select input. When you use it with an HID (human-interface device), the U421 USB controller can transfer data at rates as high as 800 bytes/sec. With additional filtering to reduce noise, the USB port provides 5V power to the circuit.

The MAX1168's sample-and-hold circuit must acquire the input voltage and charge its 45-pF holding capacitor in 3 µsec and thus requires a fast amplifier to minimize acquisition errors. Available in dual and quad versions, the MAX4230 provides a 10-MHz bandwidth, 2V/µsec slew rate, rail-to-rail inputs and outputs, and the ability to operate from a 5V rail or from voltages as low as 2.7V. The MAX4230's bias current—typically, 50 pA—allows significant input impedance without affecting accuracy.

To provide protection from overvoltages and apply input-voltage scaling, each buffer amplifier's input includes a 100-kΩprecision-matched resistive divider. This application uses Maxim's MAX5490VA10000 10-to-1 dividers, which provide a scaling factor of 1/11, to allow maximum readable inputs of 45V at resolutions of 687.5 µV.