DIO2388A/B

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DIO2388A/B• Rev. 1.6

Figure 13. Basic circuit for shutting down DIO2388X with grounded reference

Note that there is typically slightly more than 1 MΩ impedance (from the combination of 1 MΩ feedback and 5 kΩ

input resistors) from each input of the DIO2388X to the OUT pin and to the REF pin. The amount of current

flowing through these pins depends on the respective ultimate connection. For example, if the REF pin is

grounded, the calculation of the effect of the 1 MΩ impedance from the shunt to ground is straightforward.

However, if the reference or op amp is powered while the DIO2388X is shut down, the calculation is direct;

instead of assuming 1 MΩ to ground, however, assume 1 MΩ to the reference voltage. If the reference or op

amp is also shut down, some knowledge of the reference or op amp output impedance under shutdown

conditions is required. For instance, if the reference source behaves as an open circuit when it is unpowered,

little or no current flows through the 1 MΩ path.

Regarding the 1 MΩ path to the output pin, the output stage of a disabled DIO2388X does constitute a good path

to ground; consequently, this current is directly proportional to a shunt common-mode voltage impressed across

a 1 MΩ resistor.

As a final note, when the device is powered up, there is an additional, nearly constant, and well-matched 25 μA

that flows in each of the inputs as long as the shunt common-mode voltage is 3 V or higher. Below 2 V common-

mode, the only current effects are the result of the 1 MΩ resistors.

REF input impedance effects

As with any difference amplifier, the DIO2388X series common-mode rejection ratio is affected by any

impedance present at the REF input. This concern is not a problem when the REF pin is connected directly

to most references or power supplies. When using resistive dividers from the power supply or a reference

voltage, the REF pin should be buffered by an op amp.

In systems where the DIO2388X output can be sensed differentially, such as by a differential input

analog-to-digital converter (ADC) or by using two separate ADC inputs, the effects of external impedance on

the REF input can be cancelled. Figure 14 depicts a method of taking the output from the DIO2388X by using the

REF pin as a reference.