Circuit functions and advantages

The circuit shown in Figure 1 is a configurable 4 mA to 20 mA loop-powered transmitter based on an industry-leading micropower instrumentation amplifier. The total error without adjustment is less than 1%. A switch can be used to configure either a transmitter that converts a differential input voltage to a current output (Figure 1) or a receiver that converts a 4 mA to 20 mA current input to a voltage output (Figure 5).

The design is optimized for precision, low noise and low power industrial process control applications. When operating as a transmitter, the circuit can accept input voltages from 0 V to 5V or 0 V to 10 V. When used as a receiver, it can provide an output voltage of 0.2 V to 2.3 V or 0.2 V to 4.8 V, compatible with ADCs using 2.5 V or 5 V reference sources. The supply voltage range is 12 V to 36 V when operating as a transmitter and 7 V to 36 V when operating as a receiver.

Because the circuit is configurable, a single hardware design can serve as both a backup transmitter and a backup receiver, reducing customer inventory requirements.

Circuit description

The circuit is equipped with the AD8420 , an instrumentation amplifier based on an indirect current feedback architecture. Thanks to this architecture, the AD8420 has excellent input and output characteristics. Unlike traditional instrumentation amplifiers, the AD8420 can easily amplify signals at or just below ground without requiring dual power supplies. The AD8420 has a rail-to-rail output voltage swing that is completely independent of the input common-mode voltage. This frees the AD8420 from the limitations caused by the interaction between the common-mode input and output voltages found in most traditional instrumentation amplifier architectures. Flexible input and output features, coupled with micropower consumption (80 μA maximum at 0 V input) and wide supply range, make the AD8420 ideal for flexible, low-power industrial applications.

Transmitter configuration

Figure 1 shows a simplified circuit diagram of a 4mA to 20mA transmitter configuration. The circuit consumes only about 1 mA, making it ideal for loop-powered applications. The input range of the transmitter is 0 V to 5 V and 0 V to 10 V, selectable via jumper P3. Then, adjust the input voltage range to 0.195 V to 0.990 V because the differential input voltage of the AD8420 is limited to ±1 V.

The input of the AD8420 has a differential mode noise filter (40 kΩ/3.3 nF) with a bandwidth of 1.2 kHz and a common mode noise filter (20 kΩ/330 pF) with a bandwidth of 24 kHz.

The AD8420's indirect current feedback architecture forces the amplifier's differential input voltage to appear between its FB and REF pins. Transistor Q1 then converts the voltage range of 0.195 V to 0.990 V into a current of 3.9 mA to 19.8 mA flowing through the R9 50 Ω sense resistor.

The current through the R9 sense resistor includes the circuit current and the Q1 current, but does not include the AD8420 current I _AMP .

The AD8420's unique architecture makes its supply current predictable, ranging from 100 μA to 200 μA when an input voltage of 0.195 V to 0.990 V is applied between +IN and −IN. This supply current increases the current through R9, increasing the total output loop current to 4 mA to 20 mA. Therefore, the total current in the loop is given by:

For the circuit to operate properly, the total circuit supply voltage must be greater than 7 V to provide adequate headroom for the ADR02 reference.

The loop supply voltage is also limited to 36 V (maximum). An advantage of the AD8420 is its high-impedance reference pin, which does not require an additional op amp to be driven, thereby reducing power, cost, and space requirements in the transmitter circuit. For the loop to function properly, board ground and loop ground must not be connected unless R9 (50 Ω sense resistor) is required.

Adjustable resistor selection

The differential input voltage range of the AD8420 is limited to ±1V (max). Therefore, in order to accept the higher industrial input voltage range, the circuit uses a trim resistor network to convert the 0 V to 5 V or 0 V to 10 V input to 0.195 V to 0.990 V. The following equations use nodal analysis to find the values ​​of R1, R2, R3, and R4 in the circuit:

Similarly, find the value of the corresponding adjustment resistor:

The resistors provided with the EVAL-CN0314-EB1Z board are as follows: In the actual circuit, the 0.1% resistor closest to the EIA standard must be selected, so a fixed offset error can be obtained.

With these values ​​provided with the board, the offset error due to these resistor values ​​can be calculated using the following equation:

For a 0 V to 5 V input, VREF = 5 V, R1 = 5.05 kΩ, R2 = 20.5 kΩ, and R5 = 1 kΩ.

To minimize this offset error, a combination of two 0.1% resistors can be used to more closely approximate the calculated resistance value.

Total transmitter circuit accuracy

A reasonable approximation of the total error due to resistor tolerance is to assume that each critical resistor contributes equally to the total error. The four key resistors are R1 or R3, R2 or R4, R5, and R9. The worst-case tolerance due to the 0.1% resistor can result in a maximum total resistance error of 0.4%. If the rss error is assumed, the total rss error is 0.1√4 = 0.2%.

The maximum error and rss error due to active components in the system (AD8420 uses level A and ADR02 uses level B) are shown in the table below.

Adding the worst-case error due to active component offsets to the worst-case resistor tolerance error of 0.4% results in:

These errors assume the selection of an ideal resistor and assume that these errors result from its tolerance.

Actual error data for the circuit are shown in Figures 3 and 4, where the loop supply voltage = 25 V. The total output error (%FSR) is calculated by dividing the difference between the measured and ideal output current by the FSR (16 mA) and multiplying the result by 100.

Receiver configuration

Figure 4 shows a simplified receiver configuration. The receiver circuit converts the current signal into a voltage level that is compatible with most single-ended input ADCs that use a 2.5 V or 5 V reference.

Resistor R6 is used to detect the 4 mA to 20 mA signal and convert it into an input voltage of 0.2 V to 1 V for the amplifier. The input voltage is then reflected through the amplifier's FB and REF pins. Unlike most direct gain receivers, which have an output voltage range of 1 V to 5 V, the circuit uses an ADR02 and gain and trim resistors to increase the output range of 0.2 V to 4.8 V. This maximizes the input dynamic range of the ADC using a 5 V reference. The resulting extra headroom provides guaranteed linearity over the entire input signal range. The receiver circuit can also be configured using jumper P4 to provide an output voltage of 0.2 V to 2.3 V for the ADC using a 2.5 V reference.

Unlike many other single-supply instrumentation amplifiers, a key advantage of the AD8420 for this application is that it can sense near-ground currents without exceeding the input range or encountering common-mode limitations. In addition, the AD8420 also has gain and level conversion capabilities, so it does not waste the ADC input range like a simple resistor divider.

Gain and trim resistor selection

The gain of the AD8420 is generally set by the ratio of two resistors (R11 and R10). However, the circuit uses an ADR02 to take advantage of the 20% extra range wasted by the low-end direct gain. The following equations show how to obtain the gain and trim resistor values ​​for the target output voltage range.

The resistors provided on the board are those with values ​​closest to the EIA standard 0.1% and can be obtained from the supplier. So the actual values ​​of the resistors shipped with the board are as follows:

Based on these values ​​provided by the board, the error due to the resistor value can be calculated as follows:

It is also possible to use a combination of two 0.1% resistors to get closer to the calculated value, thereby minimizing this offset error.

Total receiver circuit accuracy

A reasonable approximation of the total error due to resistor tolerance is to assume that each critical resistor contributes equally to the total error. The four critical resistors are R11 or R13, R12 or R14, R6, and R10. The worst-case tolerance due to the 0.1% resistor can result in a maximum total resistance error of 0.4%. If the rss error is assumed, the total rss error is 0.1√4 = 0.2%.

The maximum error and rss error due to active components in the system (AD8420 uses level A and ADR02 uses level B) are shown in the table below.

Adding the worst-case resistor tolerance error of 0.4% to the worst-case error due to active components, we get:

Full Scale Error = 0.4% + 0.07% = 0.47%

These errors assume the selection of an ideal resistor and assume that these errors result from its tolerance.

Actual error data for the receiver circuit is shown in Figures 5 and 6, where V _CC = 25 V. The total output error (%FSR) is calculated by dividing the difference between the measured output voltage and the ideal output current by the FSR of the output voltage and multiplying the result by 100.

For a design support package for the EVAL-CN0314-EB1Z transmitter/receiver board, including complete schematics, bill of materials, and layout files, please refer to: http://www.analog.com/CN0314-DesignSupport .