Analog circuit time constant compensation method for a digital transmitter using an analog output

1. A process transmitter comprising:

a circuit producing a plurality of digital values representing magnitudes for an analog signal;

a filter receiving the plurality of digital values and producing a plurality of filtered digital values;

output analog circuitry configured to receive the plurality of filtered digital values and output an analog signal on a communication channel of the process transmitter, wherein the output analog circuitry has a transfer function and wherein the filter has a transfer function that at least partially offsets the transfer function of the output analog circuitry; and

wherein the filter comprises a clipping function to limit magnitudes of the filtered digital values to a range of magnitude values supported by the output analog circuitry and an analog circuitry output emulator that estimates a latest output of the output analog circuitry based on a previous output of the clipping function.

2. The process transmitter of claim 1 wherein the transfer function of the filter has a pole that is a function of an update time period of the output analog circuitry.

3. The process transmitter of claim 2 wherein the filter comprises an all-pass digital filter.

4. The process transmitter of claim 1 wherein the filter further comprises a filter emulator that uses the estimate of the latest output of the output analog circuitry to emulate an all-pass digital filter.

5. The process transmitter of claim 4 wherein the filter emulator uses a difference between a latest digital value and the estimate of the latest output of the output analog circuitry to determine a filtered digital value.

6. The process transmitter of claim 5 wherein the filter emulator uses a circuit time constant and an update time period for the output analog circuitry to determine the filtered digital value.

7. A process transmitter comprising:

a sensor providing a sensor signal representative of a process variable;

circuit elements providing a plurality of digital values based on the sensor signal;

a filter filtering the plurality of digital values to form filtered digital values;

output analog circuitry receiving the filtered digital values and producing an analog signal based on the filtered digital values wherein the output analog circuitry has a frequency-domain transfer function and wherein the filtered digital values at least partially counteract effects of the frequency-domain transfer function; and

wherein the filter comprises a clipping function to limit magnitudes of the filtered digital values to a range of magnitude values supported by the output analog circuitry and an analog circuitry output estimator to estimate a latest output of the output analog circuitry from a previous output of the clipping function.

8. The process transmitter of claim 7 wherein the filter comprises an all-pass digital filter.

9. The process transmitter of claim 7 wherein the filter further comprises a filter emulator that uses the estimate of the latest output of the output analog circuitry to emulate an all-pass digital filter.

10. The process transmitter of claim 9 wherein the filter emulator uses a difference between a latest digital value from the circuit elements and the estimate of the latest output of the output analog circuitry to determine a filtered digital value.

11. The process transmitter of claim 10 wherein the filter emulator uses a circuit time constant and an update time period for the output analog circuitry to determine a filtered digital value.

12. A method comprising

generating a digital value representing a desired magnitude for an analog output of a process transmitter;

filtering the digital value to produce a filtered digital value;

applying the filtered digital value to output analog circuitry to produce the analog output for the process transmitter wherein the output analog circuitry has a transfer function with a pole and the filter has a transfer function with a zero that is the inverse of the pole; and

wherein filtering the digital value comprises using a circuit time constant and update time period of the output analog circuitry to determine the filtered digital value and applying a clipping function to limit the magnitude of the filtered digital value to a range of magnitude values supported by the output analog circuitry.

13. The method of claim 12 wherein the desired magnitude for the analog output is within a second range of values designated for a communication channel.

14. The method of claim 12 wherein filtering the digital value comprises estimating the analog output and using the estimate of the analog output to form the filtered digital value.

15. The method of claim 14 further comprising determining the circuit time constant based on a temperature of the process transmitter.