4 Design performance metrics

There are several transmitter performance metrics to evaluate:

Loop Compliance Voltage is the range of the loop voltage at which the transmitter is functioning. It is mainly determined by LDO limits and affected by series elements within the loop, including protection devices. The typical loop compliance voltage range is 12V to 36V.

Resolution is the number of distinct current output values that the transmitter can generate and is directly linked to the DAC native resolution. Commercial 4mA to 20mA transmitters have resolutions between 12 bits and 16 bits.

Linearity error is mostly determined by the DAC’s integral nonlinearity, which is the maximum error (in least significant bits [LSBs]) over the whole output range.

Noise is measured by the root-mean-square (RMS) of output noise current. This noise can render some of the output level indistinguishable, reducing the effective resolution. Effective resolution in this context is a measure of noise performance. For 16-bit-resolution systems, effective resolution between 13 bits and 15 bits is expected, depending on signal bandwidth.

Accuracy measures the deviation of the current output from the ideal current value. This includes the RMS sum of offset errors, gain errors and nonlinearity error, plus the temperature drifts of these values. Total unadjusted error indicates the level of inaccuracy.

Dynamic performance includes signal bandwidth and transmitter stability. Bandwidth refers to the maximum current signal bandwidth that can be transmitted over the loop. This bandwidth is determined by the DAC settling time and amplifier circuit bandwidth, as well as the transconductance of the bypass transistor. Using a degeneration resistor eliminates the dependence on variation of the transistor transconductance (g_m). Often, the amplifier circuit is externally compensated as well. Stability is related to the bandwidth of the loop and compensation capacitor values. Reducing capacitance on critical nodes of the loop will ensure stability. See the DAC161S997 data sheet for a detailed analysis of loop stability and its requirements. For HART-enabled transmitters, reducing the bandwidth with external components helps prevent interference with the HART signal.

Circuit protection protects the transmitter from abnormal conditions such as reverse loop polarity and surge events. Reverse polarity is blocked by a diode. If operating the transmitter with reverse polarity, use a rectifier bridge, as shown in Figure 3. Surge protection requires a transient voltage suppressor diode (such as the TVS3301) and passive elements to limit current during high-voltage events. These protection elements require some headroom during operation, and increase the minimum compliance voltage.