LMP8480, LMP8481

SNVS829C – MARCH 1999 – REVISED SEPTEMBER 2015

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Feature Description (continued)

8.3.7 Input Series Resistance

Because the input stage uses precision resistors to convert the voltage on the input pin to a current, any

resistance added in series with the input pins will change the gain. If a resistance is added in series with an

input, the gain of that input will not track that of the other input, causing a constant gain error.

TI does not recommend using external resistances to alter the gain, as external resistors will not have the same

thermal matching as the internal thin film resistors.

If resistors are purposely added for filtering, resistance should be added equally to both inputs and the user

should be aware that the gain will change slightly. See the end of the Theory of Operation section for the internal

resistor values. External resistances should be kept below 10 ohms.

8.3.8 Minimum Output Voltage

The amplifier output cannot swing to exactly 0 V. There will always be a minimum output voltage set by the

output transistor saturation and input offset errors. This will create a minimum output swing around the zero

current reading due to the output saturation. The user should be aware of this when designing any servo loops or

data acquisition systems that may assume 0 V = 0 A. If a true zero is required, the LMP8481 should be used

with a VREF set slightly above ground (> 50 mV). See the Swinging Output Below Ground section for a possible

solution to this issue.

8.3.9 Swinging Output Below Ground

If a negative supply is available, a pulldown resistor can be added from the output to the negative voltage to

allow the output to swing a few millivolts below ground. This will now allow the ADC to resolve true zero and

recover codes that would normally be lost to the negative output saturation limit.

Figure 26. Output Pulldown Resistor Example

A minimum of 50 µA should be sourced (“pulled”) from the output to a negative voltage. The pulldown resistor

can be calculated from:

(8)

For example, if a –5-V supply is available, a pulldown resistor of 5 V/50 µA = 100 k

Ω should be used. This will

allow the output to swing to about 10 mV below ground.

This technique may also reduce the maximum positive swing voltage. Do not forget to include the parallel loading

effects of the pulldown any output load. TI recommends not to exceed –100 mV on the output. Source currents

greater than 100 µA should be avoided to prevent self-heating at high-supply voltages. Pulldown resistor values

should not be so low as to heavily load the output during positive output excursions. This mode of operation is

not directly specified and is not ensured.

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