Datasheet ADVFC32 (Analog Devices) - 6

ADVFC32
application of Figure 2, a 10 ppm/°C input resistor used with a the nonlinearity of the ADVFC32 is only 0.05% maximum 100 ppm/°C capacitor may result in a maximum overall circuit (0.01% typ), the resolution is much higher, allowing it to be gain drift of: used in 16-bit measurement and control systems where a mono- tonic transfer function is essential. The resolution of the circuit 100 ppm/°C (ADVFC32BH) + 100 ppm/°C (C1) shown in Figure 5 is dependent on the amount of time allowed + 10 ppm/°C (RIN) = 210 ppm/°C to count the ADVFC32 frequency output. Using a full-scale Although RIN and C1 have the most pronounced effect on tem- frequency of 100 kHz, an 8-bit conversion can be made in about perature stability, the offset circuit of resistors R4 and R5 may 10 ms, and a 2 second time period allows a 16-bit measurement, also have a slight effect on the offset temperature drift of the including offset and gain calibration cycles. circuit. The offset will change with variations in the resistance of As shown in Figure 5, the input signal is selected via the AD7590 R4 and supply voltage changes. In most applications the offset input multiplexer. Positive and negative references as well as a adjustment is very small, and the offset drift attributable to this ground input are provided to calibrate the A/D. This is very circuit will be negligible. In the bipolar mode, however, both the important in systems subject to moderate or extreme temperature positive reference and the resistor used to offset the signal range changes since the gain temperature coefficient of the ADVFC32 will have a pronounced effect on offset drift. A high quality refer- is as high as ± 150 ppm/°C. By using the calibration cycles, the ence and resistor should be used to minimize offset drift errors. A/D conversion will be as accurate as the references provided. Other circuit components do not directly influence temperature The AD542 following the input multiplexer provides a high performance as long as their actual values are not so different impedance input (1012 ohms) and buffers the switch resistance from nominal value as to preclude operation. This includes from the relatively low impedance ADVFC32 input. integration capacitor C2. A change in the capacitance value of If higher linearity is required, the ADVFC32 can be operated at C2 results in a different rate of voltage change across C2, but 10 kHz, but this will require a proportionately longer conversion this is compensated by an equal effect when C2 is discharged time. Conversely, the conversion time can be decreased at the by the switched 1 mA current source so that no net effect occurs. expense of nonlinearity by increasing the maximum frequency to The temperature effects of the components described above are as high as 500 kHz. the same when the ADVFC32 is configured for negative or bipolar input ranges, or F/V conversion.
HIGH NOISE IMMUNITY, HIGH CMRR ANALOG DATA LINK OTHER CIRCUIT CONSIDERATIONS
In many applications, a signal must be sensed at a remote site The input amplifier connected to Pins 1, 13, and 14 is not a and sent through a very noisy environment to a central location standard operational amplifier. Although it operates like an op for further processing. In these cases, even a shielded cable may amp in most applications, two key differences should be noted. not protect the signal from noise pickup. The circuit of Figure 6 First, the bias current of the positive input is typically 40 nA provides a solution in these cases. Due to the optocoupler and while the bias current of the inverting input is ± 8 nA. Therefore, voltage-to-frequency conversion, this data link is extremely any attempt to cancel input offset voltage due to bias currents insensitive to noise and common-mode voltage interference. For by matching input resistors will create worse offsets. Second, the even more protection, an optical fiber link substituted for the output of this amplifier will sink only 1 mA, even though it will HCPL2630 will provide common-mode rejection of more than source as much as 10 mA. When used in the F/V mode, the several hundred kilovolts and virtually total immunity to electrical amplifier must be buffered if large sink currents are required. noise. For most applications, however, the frequency modulated signal has sufficient noise immunity without using an optical fiber
MICROPROCESSOR OPERATED A/D CONVERTER
link, and the optocoupler provides common-mode isolation up With the addition of a few external components the ADVFC32 to 3000 V dc. can be used as a ± 10 V A/D microprocessor front end. Although Figure 5. High Resolution, Self-Calibrating, Microprocessor Operated A/D Converter REV. B –5–