HI All,
I need to implement a high side shut resistor based current sensing with a Differential ADC. Here the line consists of around 12VDC @ current varies from 1 to 10A. Eventhough the drop stands within the Differential i/p voltage limit(+/- 2.048V), its common mode voltage is higher than what is allowed.
Also I am aware that, there are some dedicated ICs and Opamp based instrumentation amplifiers are available, I am just trying to implement with same ADC & less components (as shown in the attached image.) . Simulation everything seems good, but before implementing I am seeking some advice from you. Whether this is a good practice for implementation. Hoping for your valuable suggestions.
Thanks in advance
Regards,
Geo
Yes LM324 have a very high i/p voltage range of ~32V. Which is enough for normal cases. What I thought is just to eliminate the usage of an OPAMP, if the ADC direct interfacing as shown in the circuit is working. In simulation it seems fine, still confused regarding its implementation. No where I have seen such a kind of circuit for reference.
Thank you
Geo
Yes, for 10A it will drop ~0.50V across it. Maybe we reduce the shunt resistor value for reduce the drop. In this case it can be adjusted.
In the application circuit, there are used it for the small battery sensing, whose common mode voltage is very less than its rated values. (Its mentioned in one of its App Notes.
The max. allowable voltage to its pins is
• Input voltage < VDD+0.3V
• Input voltage > VSS-0.3V
And ADC's sensing range is within the VREF (in this case a fixed internal VREF of 2.048V). So we can't give more than that. In the differential mode , the input voltage range should be within +/- 2.048V
–VREF ≤ (VIN • PGA) ≤ (VREF – 1LSB)
Where:
VIN = CHn+ - CHn-
VREF = 2.048V
& in this case ~ 500mV is applied which is safe.
But its common rail amplitude become to 12V which can turn on the input ESD protection diodes, and result in input leakage current, causing conversion errors, or permanently damage the device.
So in order to minimize this voltage , i put a voltage divider to the both differential legs. (now for this 12V, it comes around ~950mV accordingly. )
Hi,
Yes, this is what I tried to do. Just to put a potential divider. But I don't know whether its commonly using this implementation. I didn't find a relevant reference regarding this kind of circuit. Before implementing in circuits , i am seeking for some advices. (Simulation is showing good . !)
Also I selected the higher resistor values to reduce the current consumed by each resistor which may cause its self heating.
Hi,
Yes, this is what I tried to do. Just to put a potential divider. But I don't know whether its commonly using this implementation. I didn't find a relevant reference regarding this kind of circuit. Before implementing in circuits , i am seeking for some advices. (Simulation is showing good . !)
Also I selected the higher resistor values to reduce the current consumed by each resistor which may cause its self heating.
Ok , I thought if a small resistor suppose 12K may cause a small power dissipation (~mW) lead to a small heat across it. This heat can cause some resistance changes accordingly (temp. coefficient). I thought this resistance can be make some value changes to actual parameters. May be its just a negligible fact. I just applied it here , because somewhere I have read these kind of info for the Power Supply Regulator's feed backs.
The device may run from room temperature to somewhere near to 60C. (There are 2 switching MOSFETs which is a buck-boost supplying i/p to this Current sensor. )
The problem with your high values is that they reduce the sensitivity of the bridge.
Thank you Eric for sharing the caparison of both circuits. Both graph showing a very good response. In addition to that , if we consider the same system for a bi-directional current monitoring system (Suppose a battery charging & discharging ),
I believe that, its polarity will reverse , and if its connected to the differential ADC, it will just denote by a -ve sign along with the converted ADC data.
For the LM324 based circuit, I think a -ve supply rail is required. So I believe it will be an addon in case of the dual polarity current sensing.
If you plan to use the MCP3424 with bi-directional input of the Vdiff and 16bits, I guess you know that you will only get +/-32768 either side of zero input.
If you use a Vref of say +2.5V , then [0.8V/2.5] *32768 = 10485 decimal max [28f5 hex]
This means at 10Amps its very roughly ~ 1mA/bit resolution, which I would say will be OK.
Yes Eric,
It was very clear. For this ADC, eventhogh the VREF is 2.048, for differential mode is from -2.048 to +2.048V ie , 4.096. So for the step size its 4.096/65536 = 62.5μV.
Suppose 100mA is given to ADC , ADC will generate 1600 as output result.
ie , for each 6.25mA current each bit is added.