| Electronics > Beginners |
| High side current sense using differential opamp |
| << < (3/5) > >> |
| npelov:
Well I can use 2N7002 to pull down (a BJT can't pull down closely to zero). So the output impedance will be a combination of Rdson and R2 depening on how much the trnasistor is turned on. With the transistor off the output impedance is 2k, but it shouldn't go higher than 2.5V so Rdson should be the dominating resistance here. |
| npelov:
@StillTrying :) Well The idea was to use "common emiter", but then I searched for other people doing it and I read about common drain. However they used negative supply, so it never goes near the negative rail. So I get back to common source (if that's a thing :)) @Sudo_apt-get_install_yum - First of all great nick name. I don't understand these debian guys :-DD. I don't need accuracy, but if I did maybe current-source type ICs are not the best idea. Using the drop on a resistor has it's disadvantages. I ordered ZXCT1107SA-7 (as a current mode) and CS30BL as a current to voltage converter because it has a buffer on the output. The problem is that it's fixed gain and there is shortage of gain of 20. For gain of 50 or 100 I have to use lower than 0.1 resistor which increases the expenses for resistor. Not that much, but it all adds up when I have 2 or 3 current senses. I think it'll be 2 - charge current and load current. |
| npelov:
One thing is that I did want to use high side shunt because I had arrangement like this: And you always have negative voltage on one shunt. I don't know why I though I really need negative supply to use inverting opamp. But if positive input is at ground then negative should be at 0 even if shunt voltage is negative and I still get positive output: So the whole high side could be avoided in this case (I don't really need common ground between battery, charger and load), but since I had a lot of problems before with high side current sensing I want to figure this out once and for all and never had to think about it again whenever I need high side current sensing. |
| Zero999:
--- Quote from: npelov on June 21, 2018, 08:52:28 am ---Well I can use 2N7002 to pull down (a BJT can't pull down closely to zero). So the output impedance will be a combination of Rdson and R2 depening on how much the trnasistor is turned on. With the transistor off the output impedance is 2k, but it shouldn't go higher than 2.5V so Rdson should be the dominating resistance here. --- End quote --- You have a lot of gain there, so frequency compensation is going to be difficult. How about using a current mirror with two BJTs/MOSFETs, to pull the output down? |
| StillTrying:
--- Quote from: npelov on June 21, 2018, 08:52:28 am ---So the output impedance will be a combination of Rdson and R2 depening on how much the tranasistor is turned on. With the transistor off the output impedance is 2k, but it shouldn't go higher than 2.5V so Rdson should be the dominating resistance here. --- End quote --- The large amount of gain and negative feedback makes the output impedance much lower than that, as long as there's still a reasonable amount of current flowing through the pull up or pull down resistor. I've just measured the output impedance of my simulation with the 8k2 pull up, at 600mV output, supplying +/- 1mA at 1kHz the output impedance was ~0.025 ohm. |
| Navigation |
| Message Index |
| Next page |
| Previous page |