AN840 Datasheet, PDF (3/4 Pages) Silicon Laboratories – Redefining a New State-of-the-Art in Microampere Current-Sense Amplifiers

AN840
To achieve their very-low VOS performance over temperature, over wide VSENSE voltages, and over wide power
supply voltages, higher-performance CSA incorporate chopper stabilization into the input stage, a commonly-used
technique to reduce significantly amplifier VOS. In reducing the CSAs’ VOSs to 30 µV (typically) or less, load
currents can be resolved to 12-bit resolution or better for full-scale VSENSE voltages equal to and larger than
123 mV. When compared to similar CSAs that exhibit VOSs > 100 µV or more, load current measurements are two
times more accurate using CSAs that have implemented chopper-stabilized input stages.
3.1. The CSA’s SIGN Output Comparator
As was shown in Figure 2, the bi-directional CSA incorporated one additional feature – an analog comparator the
inputs of which monitor the internal amplifier’s differential output voltage. While the voltage at its OUT terminal
indicates the magnitude of the load current, the SIGN comparator output indicates the load current’s direction. The
SIGN output is a logic high when M1 is conducting current (VRS+ > VRS). Alternatively, the SIGN output is a logic
low when M2 is conducting current (VRS+ < VRS-).
Note that, unlike other bi-directional CSAs where hysteresis was purposely introduced to prevent comparator
output voltage chatter, SIGN comparator exhibits no “dead zone” at ILOAD switchover. With respect to a 50 mV
external sense resistor, the load current transition band is less than ±0.2mA. Other types of CSAs that also utilize
an analog OUT/ comparator SIGN arrangement exhibit a SIGN transition band that can range up to 2 mV (or
40 mA referred to a 50 mV sense resistor). On this attribute alone, low-transition band, bidirectional CSAs can be
200 times more sensitive.
3.2. Internal Noise Filters
To counter the effects of externally-injected differential and common-mode noise prevalent in any load current
measurement scheme, it’s always been good engineering practice to add external low-pass filters (LPFs) in series
with the CSA’s inputs. In the design of discrete CSAs, resistors used in the external LPFs were incorporated into
the circuit’s overall design so errors because of any input-bias current-generated voltage and gain errors were
compensated.
With the advent of monolithic CSAs, utilizing external LPFs in series with the CSA’s inputs only introduces
additional offset voltage and gain errors. To minimize/eliminate the need for external LPFs and to maintain low
offset voltage and gain errors, higher-performance uni-directional and bi-directional CSAs incorporate internal
LPFs to further save system cost and improve overall system performance.
3.3. Additional Applications Tips
For optimal VSENSE, all parasitic PCB track resistances to the sense resistor should be minimized. Kelvin-sense
pcb connections between RSENSE and the CSAs’ RS+ and RS- terminals are strongly recommended. The PCB
layout should be balanced and symmetrical to minimize wiring-induced errors. In addition, the pcb layout for
RSENSE should include good thermal management techniques for optimal RSENSE power dissipation.
A 22 nF to 100 nF good-quality ceramic capacitor from the OUT terminal to GND forms a LPF with the CSAs’
ROUT and should be used to minimize voltage droop (holding VOUT constant during the sample interval). Using a
capacitor on the OUT terminal will also reduce the CSAs’ small-signal bandwidth as well as band-limiting amplifier
noise.
In conclusion, a new state of the art in CSA technology has been redefined. Novel CSAs are extremely easy to
use, can resolve charging or discharging currents with 12-bit or better resolution, exhibit very low VOS and gain
match errors, are self-powered, and consume very little supply current. These higher-performance CSAs mate
their electrical performance with pcb-space saving packages (such as SOT23-5 and SOT23-6), are specified to
operate over wide or extended industrial temperature ranges, and can operate from 2 to 25 V (and higher) power
supplies. See the documentation for the TS1100 and TS1101 Current-Sense Amplifiers. For additional information,
contact Silicon Labs.
Rev. 1.0
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