C1

0.1 PF

ADC

RSN

RSP

GND

LMP848x

50 µA

RPD = -VS / 50 µA

-VS

LMP8480, LMP8481

SNVS829A – JUNE 2012 – REVISED AUGUST 2012

www.ti.com

MINIMUM OUTPUT VOLTAGE

The amplifier output cannot swing to exactly zero volts. 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 0V = 0A. If a true zero is required, the LMP8481 should be used with

a VREF set slightly above ground (>50mV). See the SWINGING OUTPUT BELOW GROUND section below for a

possible solution to this issue.

SWINGING OUTPUT BELOW GROUND

If a negative supply is available, a pull-down 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 15. Output “Pull-Down” Resistor Example

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

be calculated from:

(10)

For example, if a -5V supply is available, a pull-down resistor of 5V/50uA = 100K should be used. This will allow

the output to swing to about 10mV 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. It is recommended not to exceed -100mV on the output. Source currents

greater than 100uA 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 guaranteed.

MAXIMUM OUTPUT VOLTAGE

The LMP8481 has an internal precision 14V low dropout regulator which limits the maximum amplifier output

Care should be taken if the output is driving an A/D input with a maximum A/D maximum input voltage lower than

the amplifier supply voltage, as the output can swing higher than the planned load maximum due to input

transients or shorts on the load and overload or possibly damage the A/D input.

A resistive attenuator, as shown in Figure 16 below, can be used to match the maximum swing to the input range

of the A/D.

16

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