**Introduction**
Accurate high-end measurement of microampere-level currents demands a small-value shunt resistor and an amplifier with low offset voltage and ultra-low power consumption. This design provides a 280μA supply current, allowing the circuit to sense currents ranging from 100μA to 250mA. This minimizes power loss across the shunt resistor while maximizing the available power for the load. The LTC2063’s rail-to-rail input allows operation at very low load currents, ensuring the input common-mode voltage remains within range. Additionally, its built-in EMI filter protects against RF interference in noisy environments. For a given sensed current, the output voltage is calculated using the formula:
**Zero Point**
The zero point, or the error current produced when no current is detected, is a critical factor in current sensing accuracy. It is typically determined by the amplifier’s input offset voltage divided by RSENSE. The LTC2063 has a very low input offset voltage (1μV typical, 5μV maximum), along with minimal input bias and offset current (1–3pA). This results in a zero-point error of only 10μA (1μV / 0.1Ω) under normal conditions, up to 50μA at maximum. As shown in Figure 2, this ensures linearity even at the lowest current level (100μA), preserving resolution without distortion. The output voltage vs. current curve remains linear across the entire sensing range.
Another source of zero-point error comes from the drain current (IDSS) of the output PMOS transistor when it is nominally off. A MOSFET with high IDSS can produce a non-zero VOUT even when no current is present. In this design, the Infineon BSP322P transistor used has a maximum IDSS of 1μA at |VDS| = 100V. At room temperature and VDS = -7.6V, the typical IDSS is just 0.2nA, resulting in an error of only 1μV or 100nA equivalent input current.
**Circuit Design**
The LT1389-4.096 reference, combined with a bootstrap circuit consisting of M2, R2, and D1, creates a low-power, isolated 3V rail (approximately 4.096V + M2 VTH, typically -1V). This ensures the LTC2063 is protected from supply voltages above 5.5V. While a series resistor could provide a bias current, using transistor M2 allows for a higher overall supply voltage while keeping current consumption to only 280μA at the upper end.
**Accuracy**
The LTC2063’s input offset voltage introduces a fixed error of 10μA. At 250mA full scale, this results in an error of just 0.004%. However, at the lower end (100μA), the same 10μA error represents 10% of the measured value. Since the offset is constant, it can be calibrated. Figure 3 shows that the total offset, including parasitic thermocouples and input resistance mismatch, is only 2μV.
The gain in Figure 3 is 100.05 V/V, slightly higher than the expected 98.77 V/V (based on 4.978k / 50.4). This discrepancy may be due to different temperature coefficients between RDRIVE and RIN.
Noise is the main contributor to output uncertainty, so filtering with large parallel capacitors is essential to reduce noise bandwidth and total noise. With a 1.5Hz output filter, the LTC2063 produces about 2μVP-P of low-frequency noise. Averaging over time further reduces noise-related errors.
Other error sources include parasitic board resistance in series with RSENSE, tolerance in gain-setting resistors RIN and RDRIVE, temperature coefficient mismatches, and parasitic thermocouple voltages at the op-amp input. Using Kelvin-sense 4-pin resistors for RSENSE and 0.1% tolerance resistors with similar temperature coefficients helps minimize these errors. R1 should have the same metal terminal as RIN to counteract thermocouple effects, and asymmetrical thermal gradients should be avoided.
The total error from all sources is up to 1.4% at a 2.5V full-scale output, as shown in Figure 4.
**Supply Current**
As shown in Figure 5, the minimum supply current required for the LT1389 and LTC2063 is 2.3μA at 4.5V and 100μA, increasing to 280μA at 90V and 250mA. In addition to the active device current, the output current IDRIVE to M1 must also be supplied, which scales with the output voltage—ranging from 200μA at 1mV to 500μA at 2.5V. Thus, the total supply current ranges from 2.5μA to 780μA. Setting RDRIVE to 5kΩ ensures proper ADC drive.
**Input Voltage Range**
In this architecture, the maximum supply voltage is limited by the PMOS transistor’s maximum |VDS|. The BSP322P is rated for 100V, making 90V a safe operating limit.
**Output Range**
This design drives a 5kΩ load, making it suitable for many ADCs. The output voltage ranges from 0V to 2.5V. The LTC2063’s rail-to-rail output limits the maximum gate drive based on its headroom, which is typically 3V in this configuration.
Since the output is a current, not a voltage, ground or wire offsets do not affect accuracy. This allows for longer leads between the output PMOS M1 and RDRIVE, enabling RSENSE to be close to the current source while RDRIVE is near the ADC. However, longer leads increase EMI sensitivity. A 100nF capacitor (C3) on RDRIVE helps filter out harmful EMI before it reaches the next stage.
**Speed Limit**
With a gain-bandwidth product of 20kHz, the LTC2063 is best suited for signals up to 20Hz. A 22μF capacitor (C2) in parallel with the load filters the output noise to 1.5Hz, improving accuracy and protecting downstream stages from sudden current surges. This filtering increases settling time, especially at the lowest input current levels.
**Conclusion**
The LTC2063 offers ultra-low input offset voltage, minimal input bias and offset current, and rail-to-rail inputs, enabling accurate current measurements across the 100μA to 250mA range. Its maximum supply current is only 2μA, ensuring the overall supply current remains below 280μA for most of its operating range. Combined with low supply voltage requirements, this makes the LTC2063 ideal for applications where power efficiency and accuracy are critical.
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