2 Dual-supply circuits

Figure 1 is a simplified schematic of a discrete, dual-supply IA using the Texas Instruments (TI) TLV9064 quad op-amp circuit. In this circuit, three of the four amplifier channels (A, B and C) are connected as a traditional three-op-amp IA. The reference voltage (V_REF) connects to ground. With no use for the fourth channel, D, it is therefore connected as a buffer to mid-supply (ground) through a resistor for transient robustness. All resistors labeled “R” have a value of 10 kΩ; R_G sets the differential gain. The differential input voltage is V_IN+ − V_IN– and the output voltage is V_OUT. Some components, such as the load resistor (10 kΩ) and decoupling capacitors, are not shown. Drawing all circuits from a package perspective illustrates the number of external discrete components.

Equation 1 gives the transfer function for this circuit:

Designers will typically select a discrete IA when PCB area and performance are secondary to cost and gain range. TI’s TLV9064IRUCR op amp was selected for this comparison because it is a rail-to-rail input/output device (RRIO) with a wide bandwidth (10 MHz) and a low typical initial input offset voltage (V_OS(typ) = 300 µV), and comes in a small package (RUC = X2QFN = 4 mm²). While there are less expensive RRIO quad op amps in RUC/X2QFN packages, they come at the expense of bandwidth and typical offset voltage.

To be consistent with the design priorities of discrete IAs, inexpensive ±1% tolerance, ±100-ppm/°C drift resistors were installed. Not only do these resistors differ in initial value, they will likely drift significantly over temperature. Since R_G is external, the gain for this configuration is primarily limited by the input offset voltage of the op amps.

Figure 2 is a simplified schematic of the TI INA350ABS, a general-purpose dual-supply IA with an integrated R_G. V_REF connects to ground. This circuit integrates all resistors in the IA. The differential input voltage is V_IN+ − V_IN– and the output voltage is V_OUT. Some components, such as the load resistor (10 kΩ) and decoupling capacitors, are not shown. The gain of the IA is set based on the switch connected to pin 1 (open = 20 V/V, closed = 10 V/V). In an actual application, the switch would not be present. To enable the device, connect pin 8 (SHDN) to V+ or leave it floating.

Equation 2 gives the transfer function for this circuit:

Designers will typically select this IA when their requirements necessitate a balance of cost, performance and PCB area. The INA350ABSIDSGR IA was chosen for this comparison because of its affordability, performance, small package (lead DSG = WSON = 4 mm²), selectable gain (10 V/V or 20 V/V) and low typical input offset voltage (V_OS(typ) = 200 µV). This implementation needs no external components. For designs that require higher gains, the INA350CDS has gains of 30 V/V or 50 V/V.

Figure 3 is a simplified schematic of the TI INA333 precision dual-supply IA with an external R_G. V_REF connects to ground. In this circuit, the IA integrates all resistors except R_G. The differential input voltage is V_IN+ − V_IN– and the output voltage is V_OUT. Some components, such as the load resistor (10 kΩ) and decoupling capacitors, are not shown.

Equation 3 gives the transfer function for this circuit:

Designers typically use a precision IA when performance is the highest priority. The INA333AIDRGR precision IA was selected for this comparison because it is low voltage (5 V), has excellent precision (G = 1 V/V, V_OS(typ) = 35 µV) and comes in a small package (DRG = WSON = 9 mm²). The performance over temperature depends on the selection of the external R_G. Therefore, to be consistent with the primary design priority – performance – we used a precision R_G for a gain of 10 V/V (±0.05%, ±10 ppm/°C). Because the precision op amps are integrated, this implementation has excellent gain range (1 V/V to 1,000 V/V). The overall cost is usually greater than the other two solutions, however, given the integrated precision op amps and required precision R_G.