Low Cost, Level Shifted Low Side Current Monitor for Negative High Voltage Rails
Although it is possible to operate both the AD7171 VDD and REFIN(+) from the 3.3 V power supply, using a separate reference provides better accuracy. A 2.5 V reference is chosen to provide sufficient headroom.
Figure 2. Plot of Output and Error vs. Shunt Voltage
PCB Layout Considerations
In any circuit where accuracy is crucial, it is important to consider the power supply and ground return layout on the board. The PCB should isolate the digital and analog sections as much as possible. This PCB was constructed in a four layer stack up with large area ground plane layers and power plane polygons. See the MT-031 Tutorial for more discussion on layout and grounding and the MT-101 Tutorial for information on decoupling techniques.
Figure 3. Alternate Galvanically Isolated Negative Rail Current Sense Circuit Powered Directly from −48 V Source and ADuM5402 Isolator (All Connections and Decoupling Not Shown)
- PC with a USB port and Windows® XP or Windows Vista® (32-bit), or Windows® 7 (32-bit)
- EVAL-CN0188-SDPZ circuit evaluation board
- EVAL-SDP-CB1Z SDP evaluation board
- CN0188 evaluation software
- Power supply: +6 V, or +6 V “wall wart”
- Shunt resistor with maximum voltage of 50 mV at the maximum load current.
- Electronic load
Load the evaluation software by placing the CN0188 evaluation software disc in the CD drive of the PC. Using "My Computer," locate the drive that contains the evaluation software disc and open the Readme file. Follow the instructions contained in the Readme file for installing and using the evaluation software.
Functional Block Diagram
See Figure 1 of this circuit note for the circuit block diagram and the “EVAL-CN0188-SDPZ-SCH” pdf file for the circuit schematics. This file is contained in the CN0188-DesignSupport Package.
Connect the 120-pin connector on the EVAL-CN0188-SDPZ circuit board to the connector marked “CON A” on the EVAL-SDP-CB1Z evaluation (SDP) board. Nylon hardware should be used to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors.
Connect a shunt resistor across the input terminals (RSHUNT) with a load to ground as indicated in Figure 1. With power to the supply off, connect a +6 V power supply to the pins marked “+6 V” and “GND” on the board. If available, a +6 V "wall wart" can be connected to the barrel connector on the board and used in place of the +6 V power supply. Connect the USB cable supplied with the SDP board to the USB port on the PC. Note: Do not connect the USB cable to the mini USB connector on the SDP board at this time.
It is important to connect the system ground and the PCB isolated ground to guarantee correct voltage levels and operation. Test point 31 and test point 32 give access to the GND_ISO required to properly make this connection.
Apply power to the +6 V supply (or “wall wart”) connected to the EVAL-CN0188-SDPZ circuit board. Launch the evaluation software and connect the USB cable from the PC to the USB mini-connector on the SDP board.
Once USB communications are established, the SDP board can be used to send, receive, and capture serial data from the EVAL-CN0188-SDPZ board. Data can be recorded for various values of load current as the electronic load is stepped.
Information and details regarding how to use the evaluation software for data capture can be found in the CN0188 evaluation software Readme file.
Information regarding the SDP board can be found in the SDP User Guide.
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