q2418130103p
Newbie level 6
Hello,
The executive summary for this thread is: single supply measurements of signals with negative components.
I am currently working on a battery powered design which will be used to collect high impedance, generally ground referenced, sensor voltages from outside systems. The analog front end of this design would closely resemble an oscope: high impedance attenuation(10:1 and 100:1) , clipper, high input impedance buffer, pga, etc.
I would prefer to not split the battery supply in order to provide a virtual ground for purposes of measuring signals that go below ground. This is because I do not want to have to run my other regulators off this virtual ground (like the 3.3 and 1.8 required for fpga, mcu, pga, etc). Someone correct me if this is a flimsy reason. I have not yet calculated out power requirements, so it may really be a non issue.
This means adding a dc offset just after the signal hits the board. I have tried this a number of ways. To name a few:
1) Connect the ground of the sensor's system to a reference voltage on the battery powered system. I have tried this a couple different ways, all of which work fine but the problem is what happens if the sensor system's ground was the same ground as the computer which may be attached via jtag programmer or usb. The reference voltage would get shorted. Depedning on the attenuator arrangement it either causes damage or simply ruins the measurements.
2) Use a regular voltage divider as the attenuator, but instead of connecting it to ground connect it to a reference voltage instead. The problem with this is that the reference voltage will be attenuated by the attenuator, which means I need a seperate reference for each attenuation mode (10:1 vs 100:1).
3) Traditional summing opamp, the problem is that I dont know of a configuration that will allow me to maintain the high input impedance of the input buffer.
Currently my favorite option is to use a differential attenuator, connect one output of the attenuator to a reference voltage, send this differential signal to an instrumentation amp which then allows me to adjust the output common mode down to levels useful with a 3.3v pga. This doesnt resolve the ground path issue, and it also means I need to double the number of relays used to switch attenuators.
If I were to rank system requirements I would say: accuracy/robusteness, complexity, then cost.
Any help would be appreciated.
Thanks,
Jay
The executive summary for this thread is: single supply measurements of signals with negative components.
I am currently working on a battery powered design which will be used to collect high impedance, generally ground referenced, sensor voltages from outside systems. The analog front end of this design would closely resemble an oscope: high impedance attenuation(10:1 and 100:1) , clipper, high input impedance buffer, pga, etc.
I would prefer to not split the battery supply in order to provide a virtual ground for purposes of measuring signals that go below ground. This is because I do not want to have to run my other regulators off this virtual ground (like the 3.3 and 1.8 required for fpga, mcu, pga, etc). Someone correct me if this is a flimsy reason. I have not yet calculated out power requirements, so it may really be a non issue.
This means adding a dc offset just after the signal hits the board. I have tried this a number of ways. To name a few:
1) Connect the ground of the sensor's system to a reference voltage on the battery powered system. I have tried this a couple different ways, all of which work fine but the problem is what happens if the sensor system's ground was the same ground as the computer which may be attached via jtag programmer or usb. The reference voltage would get shorted. Depedning on the attenuator arrangement it either causes damage or simply ruins the measurements.
2) Use a regular voltage divider as the attenuator, but instead of connecting it to ground connect it to a reference voltage instead. The problem with this is that the reference voltage will be attenuated by the attenuator, which means I need a seperate reference for each attenuation mode (10:1 vs 100:1).
3) Traditional summing opamp, the problem is that I dont know of a configuration that will allow me to maintain the high input impedance of the input buffer.
Currently my favorite option is to use a differential attenuator, connect one output of the attenuator to a reference voltage, send this differential signal to an instrumentation amp which then allows me to adjust the output common mode down to levels useful with a 3.3v pga. This doesnt resolve the ground path issue, and it also means I need to double the number of relays used to switch attenuators.
If I were to rank system requirements I would say: accuracy/robusteness, complexity, then cost.
Any help would be appreciated.
Thanks,
Jay