Simple logic reduces EMI

Eugene Palatnik, BCI International, Waukesha, WI -- EDN, 8/5/1999

A current-input ADC whose primary use is measuring low-level signals from photodetectors can also measure a range of voltages using the circuit in Figure 1. The main component is a dual-input current-integrating ADC, IC₁. IC₂ and IC₃ provide the buffered 4.096V reference for IC₁. The figure does not include a µC, which typically oversees the digital control and data retrieval.

When you use IC₁ with photodetectors, this ADC integrates the currents at input pins 1 and 28 for a user-controlled integration period, T_INT. IC₁ then digitizes the output voltages of the integrators into 20-bit digital words that are ready for retrieval over the serial interface. The only modifications necessary to enable IC₁ to measure voltages are resistors in series with the inputs. IC₁'s integrators hold the input at a virtual ground, so the applied voltages produce currents through the resistors, which by Ohm's law equal V_IN/R_INPUT. The full-scale voltage input for the circuit in Figure 1 is

where I_FS is the full-scale input current; Q_FS is the full-scale range of IC₁ in coulombs, set by pins RANGE₀ to RANGE₂; and T_INT is the user-controlled integration period. R_INPUT's value should be large, preferably greater than 10 Mµ. Avoid using resistors with poor voltage coefficients and excess noise. Caddock Electronics Inc (www.caddock.com) offers a variety of high-value resistors with good performance.

Eight full-scale ranges are available in IC₁. Seven of these ranges are internal ranges of 50 to 350 pC in steps of 50 pC. The eighth range uses external capacitors C₁ to C₄ to establish a full-scale range of approximately 0.96XV_REFXC_EXT. For the 250-pF capacitors in Figure 1, this external range is approximately 866 pC. Surface-mount COG capacitors are a good choice for C₁ to C₄. The CONV Pin of IC₁ sets the integration time. In a typical configuration, the minimum integration period for continuous operation is 500 µsec, and the maximum period is 1 sec.

The circuit exhibits a number of useful features. First, the dynamic range of the circuit is large: IC₁ outputs 20-bit words. Adjusting Q_FS and T_INT provides additional dynamic range. The circuit can measure voltages that span more than seven orders of magnitude using a fixed value of R_INPUT. Second, the virtual ground at IC₁'s inputs allows you to place additional resistors in parallel, such as R_INPUT1-2 and R_INPUT2-2, to measure the sum of the applied voltages. And, because IC₁ is a dual-input ADC, the circuit can simultaneously make two independent voltage measurements. Finally, the large-value resistors in series with IC₁'s inputs allow the circuit to measure large voltages. For example, if R_INPUT=100 Mµ, then a voltage of 100V produces only a 1-µA current, which IC₁ can easily measure. When using large voltages, make sure that the resistors are rated to handle the voltages and that the pc-board layout provides the proper spacing for insulating the high-voltage lines.

At first glance, you might think that the large resistors in series with IC₁'s inputs contribute a lot of thermal noise. Fortunately, the noise that these resistors produce is usually low and decreases as the resistor values increase. To see why this surprising behavior occurs, consider two identical noise models of a resistor (Figure 2). The model in Figure 2a is probably more familiar and shows the resistor modeled as a voltage source in series with a noiseless resistor. The spectral density of the voltage noise, S_v(f), is proportional to the value of resistance. The Thevenin-equivalent model in Figure 2b comprises a current source in parallel with a noiseless resistor. In this case, the spectral density of the current noise, S_i(f), is inversely proportional to the value of resistance. The bigger the resistance, the smaller the current noise. IC₁ measures current. The current-noise contribution of the resistor is important, so bigger resistors reduce the noise. Also, the thermal noise power is independent of resistance. (For a detailed explanation of the physics behind thermal and other noise phenomenon, see Reference 1.)

You can use the following expression to calculate the rms thermal noise, where V_FS is the integrator's full-scale voltage, not the input signal's full-scale voltage, V_inFS, in the previous equation:

You must add this thermal noise to IC₁'s inherent noise to calculate the total rms noise. Figure 3 shows noise performance versus R_INPUT for values of T_INT and Q_FS. The figure includes measurement points for comparison with the calculated noise performance.

IC₁'s inputs have very high input impedances and can be susceptible to noise pickup unless you use care in laying out the circuit. Try to keep R_INPUT as close to the inputs as possible to minimize the length of the input traces. Also, shield the input leads and IC₁ with grounds wherever possible. If noise pickup is still a problem, adjusting the integration period can notch out specific frequencies. For example, setting the integration period to 16.666 msec places a notch in the frequency response at 60 Hz. (DI #2392)

REFERENCE

1. Van der Ziel, Aldert, Noise in Solid State Devices and Circuits, John Wiley and Sons, 1986.