SBOA594 August 2024 OPA2990

2 Circuit Description

Figure 2-1 shows the combined V/I circuit. The current path is formed by U1 and U2, and the voltage path is formed by U3 and U4.

Figure 2-1 Combined V/I Stage Schematic

The current path is a two-stage voltage to current converter. The transfer function is straightforward to derive:

Equation 1.

I_{O U T} = [V_{I N} \times \frac{R_{2}}{R_{1} + R_{2}} + V_{R E F} \times \frac{R_{1}}{R_{1} + R_{2}}] \times \frac{R_{5}}{R_{3} ∙ R_{4}}

If the VREFI input is open circuit, this equation reduces to:

Equation 2.

I_{O U T} = V_{I N} \times \frac{R_{5}}{R_{3} ∙ R_{4}}

The voltage path is a non-inverting amplifier, with the feedback resistors driven by a unity gain buffer instead of connecting them directly to the output node. Buffering the output reduces the current leaked to the feedback resistors, and hence reduces the error in the current output mode. Again, the transfer function for the voltage output stage can be easily derived as:

Equation 3.

V_{O U T} = V_{I N} \times [1 + \frac{R_{8}}{R_{7}} + \frac{R_{8}}{R_{9}}] {- V}_{R E F} \times \frac{R_{8}}{R_{9}}

If the VREF input is open circuit, this equation reduces to

Equation 4.

V_{O U T} = V_{I N} \times [1 + \frac{R_{8}}{R_{9}}]

If the circuit is designed for 0-10V or 0-20mA output, VREF inputs are left floating, and the simple equations are used. Assuming VIN=0 to 2.5V, Vout stage gain needs to be set to 4V/V, and current stage gain = 8mA/V. The gains are set by choosing the proper resistor values.

opamps U1 and U2, as well as U3 and U4 are dual package opamp OPA2990S. This allows the use of different supply rails for each path. The current path has VCCI, and GND as rails, while voltage path has VCC and VSS. And each path has a shutdown signal: ISHDNI, and VSHDN. OPA4990S can be also used instead of two dual opamps. In this case, only two supply rails are available for both voltage and current sections.