DIO2388A/B

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DIO2388A/B• Rev. 1.6

unidirectional applications where the highest possible accuracy is desirable at very low inputs, bias the REF pin

to a convenient value above 50 mV to get the device output swing into the linear range for zero inputs.

A less frequent case of unipolar output biasing is to bias the output by connecting the REF pin to the supply; in

this case, the quiescent output for zero input is at quiescent supply. This configuration would only respond to

negative currents (inverted voltage polarity at the device input).

Bidirectional operation

Bidirectional operation allows the DIO2388X to measure currents through a resistive shunt in two directions. In

this case, the output can be set anywhere within the limits of what the reference inputs allow (that is, between 0 V

to V+). Typically, it is set at half-scale for equal range in both directions. In some cases, however, it is set at

a voltage other than half-scale when the bidirectional current is nonsymmetrical.

The quiescent output voltage is set by applying voltage to the reference input. Under zero differential input

conditions the output assumes the same voltage that is applied to the reference input.

Input filtering

An obvious and straightforward filtering location is at the device output. However, this location negates the

advantage of the low output impedance of the internal buffer. The only other filtering option is at the device input

pins. This location, though, does require consideration of the ±30% tolerance of the internal resistances. Figure

12 shows a filter placed at the inputs pins.

Figure 12. Filter at input pins

The addition of external series resistance, however, creates an additional error in the measurement so the value

of these series resistors should be kept to 10 Ω or less if possible to reduce impact to accuracy. The internal bias

network shown in Figure 12 present at the input pins creates a mismatch in input bias currents when a differential

voltage is applied between the input pins. If additional external series filter resistors are added to the circuit, the

mismatch in bias currents results in a mismatch of voltage drops across the filter resistors. This mismatch creates

a differential error voltage that subtracts from the voltage developed at the shunt resistor. This error results in a

voltage at the device input pins that is different than the voltage developed across the shunt resistor. Without the

additional series resistance, the mismatch in input bias currents has little effect on device operation. The amount

of error these external filter resistor add to the measurement can be calculated using Equation (2) where the gain

error factor is calculated using Equation (1).

The amount of variance in the differential voltage present at the device input relative to the voltage developed at

the shunt resistor is based both on the external series resistance value as well as the internal input resistors, R3

and R4 (or RINT as shown in Figure 12). The reduction of the shunt voltage reaching the device input pins