Circuit Puts Analog Data into Excel

The interface circuit (Fig. 1) that connects the 16-bit ADS7807 analog-to-digital converter (ADC) IC to a PC's parallel port is simple. The control software forces nine of the port's signal lines to a high state, which lets the corresponding pins provide power to the circuit through 10-V resistors. Using the parallel interface's data lines to power the interface may not work for all PCs, though. If your PC cannot supply clean 4.75-V to 5.25-V DC power to the circuit, use an external source to supply the 35 mW the circuit needs. (For more information about the parallel port, see the IEEE 1284 standard,(See 1 which provides a thorough description of signals, controls, and hardware for a PC's parallel interface.)

The 74HCT04 inverter in the interface circuit provides a clean signal to a monostable (10-kV resistor and 100-pF capacitor) that produces a 150-ns logic-zero pulse for the ADC's R/C* input. When the R/C* input goes to a logic zero, the ADC starts a conversion. The short pulse ensures that the R/C* input is back at logic one before the ADC's BUSY* (conversion complete) signal goes to a logic one. The short pulse width also minimizes any digital feedthrough that might occur during a conversion.

Select a Byte
The 16-bit ADC's BYTE input selects the byte that appears at the ADC's outputs. In turn, the 74HCT157 multiplexer selects which nibble (four bits) of each byte to transmit to the parallel port. The macro controls the flow of four nibbles to the PC and reconstructs them into a 16-bit integer.

By itself, the ADS7807 has an input voltage range of ±10 V. Using an 866-kV input resistor, however, extends the range to ±200 V according to the equation:

Vin(max) = ±10 V * (1 + Rext/45.5k)

To reduce noise on your signal, place a capacitor between the ADS7807's input and ground to form an RC network with the input resistor.

To acquire voltages, select a cell or a range of cells in a spreadsheet and then execute the macro (Fig. 2), which contiguously fills each cell with a measurement. The Excel macro uses a dynamic link library routine, CUSER3.DLL, to communicate with the ADC through the PC's parallel port. (T&MW provides the DLL file for free; see "Notes," below.) Be sure to transfer the CUSER3.DLL into your main Excel directory so the macro can locate it.

Control Your PC's Ports
To keep Excel Basic--the Basic-like language built into Excel--independent of hardware, Microsoft excluded commands that control specific I/O ports. The CUSER3.

DLL circumvents this limitation and lets you transfer bytes to output ports and retrieve bytes from input ports.Typically, a PC's parallel port resides at h0378 (data), h0379 (status), and h037A (control). Check your PC's manual to be sure you have the proper addresses for the port you want to use. If necessary, change the three address definitions at the start of the macro.

When you run the macro, it initializes the control byte and starts the ADC. After a delay of at least 25 ms, the macro checks the ADC's BUSY signal. At the end of a conversion, the macro inputs the ADC's data. For example, if you select 15 cells, the macro acquires 15 readings, one after the other. The timing between conversions depends on your PC's timing and the timing overhead of Excel. My 33-MHz 486-based PC took 23 ms per conversion, and the Nyquist bandwidth was 22 Hz.

Figure 1. The circuit for the ADC interface requires only a few TTL ICs to transfer data to an Excel spreadsheet under software control.

Calculate Offset and Gain Errors
The basic circuit doesn't compensate for gain and offset errors. You can use nominal ADS7807 offset and gain errors (without external trimming) to scale the results obtained with the macro. Use the code shown in Fig. 3 with the ADSinput() macro to provide gain and offset coefficients, if you need them. To achieve the highest accuracy, add external trimmer resistors to the circuit. (See Footnote 2 below.) T&MW