8.3.1 Selection of Sense Resistor

The value chosen for the shunt resistor, R_S, depends on the application. It plays a big role in a current sensing system and must be chosen with care. The selection of the shunt resistor needs to take in account the tradeoffs in small-signal accuracy, the power dissipated and the voltage loss across the shunt itself.

In applications where a small current is sensed, a bigger value of R_S is selected to minimize the error in the proportional output voltage. Higher resistor value improves the signal-to-noise ratio (SNR) at the input of the current sense amplifier and hence gives a more accurate output.

Similarly when high current is sensed, the power losses in R_S can be significant so a smaller value of R_S is desired. In this condition it is also required to take in account also the power rating of R_S resistor. The low input offset of the LMP8640 allows the use of small sense resistors to reduce power dissipation still providing a good input dynamic range. The input dynamic range is the ratio between the maximum signal that can be measured and the minimum signal that can be detected, where usually the input offset is the principal limiting factor.

Figure 26. Example of a Kelvin (4-Wire) Connection to a Two-Terminal Resistor

The amplifier inputs should be directly connected to the sense resistor pads using “Kelvin” or “4-wire” connection techniques. The paths of the input traces should be identical, including connectors and vias, so that these errors will be equal and cancel.

8.3.2 Sense Line Inputs

The sense lines should be connected to a point on the resistor that is not shared with the main current path, as shown in Figure 26 above. For lowest drift, the amplifier should be mounted away from any heat generating devices, which may include the sense resistor. The traces should be one continuous trace of copper from the sense resistor pad to the amplifier input pin pad, and ideally on the same copper layer with minimal vias or connectors. This can be important around the sense resistor if it is generating any significant heat gradients. Vias in the sense lines should be formed from continuous plated copper and routing through connectors should be avoided. It is better to extend the sense lines than to place the amplifier in a hostile environment.

To minimize noise pickup and thermal errors, the input traces should be treated like a high-speed differential signal pair and routed tightly together with a direct path to the input pins on the same copper layer. They do not need to be "impedance matched", but should follow the same matching rules about vias, spacing and equal lengths. The input traces should be run away from noise sources, such as digital lines, switching supplies or motor drive lines. Remember that these input traces can contain high voltage, and should have the appropriate trace routing clearances to other traces and layers. Since the sense traces only carry the amplifier bias current (typically about 12µA per input at room temp), the connecting input traces can be thin traces running close together. This can help with routing or creating the required spacings.

It should also be noted that, due to the nature of the device topology, the positive input bias current will vary with V_SENSE with an extra current approximately equivalent to V_SENSE / 5 kΩ on top of the typical 12 uA bias current. The negative input bias current is not in the feedback path and will not change over V_SENSE. High or miss-matched source impedances should be avoided as this imbalance will create an additional error over input voltage.

8.3.3 Effects of Series Resistance on Sense Lines

Because the input stage uses precision 5 KΩ resistors internally to convert the voltage on the input pin to a current, any resistance added in series with the input pins will change this resistance, and thus alter 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.

It is not recommended to use external resistance to alter the gain, as external resistors will not have the same thermal matching as the internal thin film resistors. Any added resistance will severely degrade the offset and CMRR specifications. It is recommended that the total trace resistance be less than 10 ohms.

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.

8.4.1 Bias Current at Low Common Mode Voltage

At common mode voltages below +2 V, the input bias current starts to reverse and crosses through zero at about +1.8 V. This can be seen in the Input Bias Current graphs in Figure 9 through Figure 11. Negative currents on the graph show curent flow out of the input and into the load. The graphs show the current for each input, so the actual "bias" current will be twice graph value. This total current could be as high as 1mA, depending on the point of equilibrium.

While this will not affect the vast majority of applications, it may cause MOSFET switched designs with non-linear loads (like LED's or diode-isolated loads) to "float" above ground where the load leakage and bias currents attain equilibrium. A small resistor to ground (on the load supply side) can bleed-off this current.

8.4.2 Applying Input Voltage with No Supply Voltage

The full specified input common mode voltage range may be applied to the inputs while the LMP8640 power is off (V+ = 0 V). When the LMP8640 is powered off, the R_IN resistors are disconnected internally by MOSFETS and the leakage currents are very low (sub uA).

The 6 V input differential limit still applies, so at no time should the two inputs be more than 6 V apart. There are also Zener clamps on the inputs to ground, so do not exceed the input limits specified in the Absolute Maximum Ratings.

8.4.3 Driving an ADC

The input stage of an Analog to Digital converter can be modeled with a resistor and a capacitance to ground. So if the voltage source doesn't have a low impedance, an error in amplitude measurement will occur. In this case, a buffer is needed to drive the ADC. The LMP8640 has an internal output buffer able to directly drive a capacitive load up to 30 pF, or, the input stage of an ADC. It is recommended that an external low pass RC filter be added to the output of the LMP8640 to reduce the noise and limit the bandwidth of the current sense measurement.

If the supply voltage of the LMP8640 is higher than the ADC supply voltage, care should be taken to prevent the LMP8640 output from over-driving the ADC input.

This can be accomplished with a series resistance (which should be present when driving a ADC) and a clamping diode to the ADC's power supply. The diode will clamp the ADC input to a safe level. Do not completely rely on a calculated maximum output voltage. Transients or fault conditions outside the normal conditions area can cause the output to swing to a higher than expected voltage.