Test & measurement
Figure 4. The percent error remains below 1.4 per cent for the entire range of readings.
Supply Current
The minimum supply current required by the LT1389-4.096 and LTC2063 is 2.3 µA at the minimum VSUPPLY and ISENSE (4.5 V and 100 µA), up to 280 µA at maximum VSUPPLY and ISENSE (90 V and 250 mA), as shown in Figure 5. In addition to the current consumed by
the active components, an output current IDRIVE through M1 also supplied by VSUPPLY is required, proportional to the output voltage, ranging from 200 nA for a 1.0 mV output (for 100 µA ISENSE) to 500 µA for a 2.5 V output (for 250 mA ISENSE). Thus, the total supply current in addition to ISENSE ranges from 2.5 µA to 780 µA. ROUT is set at 5 kΩ for a reasonable ADC drive value.
Figure 5. Supply current increases with supply voltage but never surpasses 280 μA.
Input Voltage range
In this architecture, the maximum supply is set by the maximum |VDS| that the PMOS output can withstand. The BSP322P is rated for 100 V, so 90 V is an appropriate operating limit.
output range
This design can drive a 5 kΩ load, which makes it a suitable stage for driving many ADCs. The output voltage range is 0 V to 2.5 V. Since the LTC2063 has rail-to-rail output, the maximum gate drive is only limited by the LTC2063’s headroom. It is 3 V typically in this design, set by the
LT1389-4.096’s 4.096 V plus the –1 V typical VTH of M2. Since the output of this circuit is a current, not a voltage, ground or
lead offset does not affect accuracy. Thus, long leads can be used
between the output PMOS M1 and ROUT, allowing RSENSE to be located near the current being sensed while ROUT is near an ADC and other subsequent signal chain stages. The drawback of long leads is increased
EMI susceptibility. 100 nF C3 across ROUT shunts away harmful EMI before it reaches the next stage’s input.
46 Speed lImItS
Since the LTC2063’s gain-bandwidth product is 20 kHz, it is recommended to use this circuit to measure signals 20 Hz or slower. The 22 µF C2, in parallel with the load, filters the output noise to 1.5 Hz for improved accuracy and protects the subsequent stage from sudden current surges. The trade-off of this filtering is longer settling time, especially at the lowest end of the input current range.
ConCluSIon
The LTC2063’s ultralow input offset voltage, low IOFFSET and IBIAS, and rail-to-rail input, provide precise current measurements over the entire range of 100 µA to 250 mA. Its 2 µA maximum supply current enables the circuit to run on far less than 280 µA supply current for most of its operating range. Along with LTC2063’s low supply voltage requirements, the low supply current allows it to be powered from a voltage reference with headroom to spare.
Analog Devices
www.analog.com August 2020 Instrumentation Monthly
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