DIO2399A/B

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DIO2399A/B• Rev. 1.7

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 9). The reduction of the shunt voltage reaching the device input pins

appears as a gain error when comparing the output voltage relative to the voltage across the shunt resistor. A

factor can be calculated to determine the amount of gain error that is introduced by the addition of external series

resistance. The equation used to calculate the expected deviation from the shunt voltage to what is seen at the

device input pins is given in Equation (1):

Gain error

(

)

(

) (

) (

)

INT

S

INT

S

INT

R

R

R

1250

R

1250

R

1250

factor

×

+

×

+

×

×

=

(1)

Where:

RINT is the internal input resistor (R3 and R4), and

RS is the external series resistance.

With the adjustment factor equation including the device internal input resistance, this factor varies with each

gain version, as shown in Table1. Each individual device gain error factor is shown in Table 2.

Table 1. Input resistance

Product

Gain

RINT (kΩ)

DIO2399A

50

20

DIO2399B

100

10

Table 2. Device gain error factor

Product

Simplified Gain Error Factor

DIO2399A

000

20

)

R

(17

000

20

S

，

，





DIO2399B

000

10

)

R

(9

000

10

S

，

，





The gain error that can be expected from the addition of the external series resistors can then be calculated

based on Equation (2):

Gain error(%) = 100 - (100 × Gain error factor)

(2)

REF input impedance effects

As with any difference amplifier, the DIO2399X 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