Quote:
Originally Posted by Kevin Sevcik
Actually, I'd look move to pages 8 and 9 of the data sheet, on "Accuracy variations as a result of Vsense and common mode voltage".
This amp is apparently most accurate at sense voltages >20mv. Which equates to 20A for us. There's 3 operating cases in this range, and we're not in the worst... So it could be worse? I've attached a snapshot of an example graph. What it boils down to is that the amp goes non-linear below 20mv, and eventually lands on some positive offset voltage at the output, instead of zero. They state the worst case zero offset at the output is 300mv. It looks less linear the closer you get to 0 Vsense, so that's exciting too.
|
Quote:
The accuracy of the INA193−INA198 current shunt monitors is a function of two main variables: VSENSE (VIN+ − VIN−) and common-mode voltage, VCM, relative to the supply voltage, VS. VCM is expressed as (VIN+ + VIN−)/2; however, in practice, VCM is seen as the voltage at VIN+ because the voltage drop across VSENSE is usually small.
This section addresses the accuracy of these specific operating regions:- Normal Case 1: VSENSE ≥ 20mV, VCM ≥ VS
- Normal Case 2: VSENSE ≥ 20mV, VCM < VS
- Low VSENSE Case 1: VSENSE < 20mV, −16V ≤ VCM <0
- Low VSENSE Case 2: VSENSE < 20mV, 0V ≤ VCM ≤ VS
- Low VSENSE Case 3: VSENSE < 20mV, VS < VCM ≤ 80V
|
So we're in Low VSENSE Case 3, where VCM is greater than the 5v supply voltage? That must be why they measure current on the high side.
Above 20A, then, this current measurement is incredibly accurate; within %0.001 if your battery is at 11.5v or greater.
Quote:
Originally Posted by Kevin Sevcik
Oh yeah. And both Jags record a zero offset for the current ADC on power up, and then subtract that from current readings forever after. So if you end up with the worst case 300mv offset, then your readings above 20A will read 15A low. We're probably not getting this worst-case scenario, but that's obviously going to affect the accuracy of ALL the current readings. Better might be to just assume/hope that you have little absolute error in the ADC, and just use the zero offset as a linear correction below 20mv Vsense.
|
Except for that. They just made it measure low for the WHOLE range. So that means, if you get a reading of 5A or greater, you need to add 15A to make it accurate. XP
Wait a minute. Where did you get the 15A? I see 0.3v on the graph, which equates to 30A.