Monday, December 13, 2010

High-Side Current Sensing with Wide Dynamic Range: Three Solutions

Precision current sensing allows designers to measure motor torque, dc-to-dc converter efficiency, bias current in a power transistor, and other critical parameters in the presence of high common-mode voltages. This article describes three solutions—discrete, integrated, and application-optimized—that provide high-accuracy, high-resolution current sensing for a variety of applications.

4 Comments:

At 3:49 PM, Anonymous Anonymous said...

In the second application, figure 2b, you mention that to utilize the entire output range to measure a unidirectional current flow, then an external source may be used to set the range. I understand the theory here, but the external source is providing (5V * 0.5kOhms)/(0.5kOhms + 20kOhms) = 0.123V
How is this useful? Can you describe in more detail how the use of an external source with both Vrefs connected at the same potnetial allows the utilization of the full output range of the AD8210?

 
At 5:01 PM, Anonymous Anonymous said...

Hello. As with all current sensors the output is not fully rail to rail. For the AD8210 we specify a min output range of 100mV. This is to take into account the temp drift of the output transistors. So, if you have 4mV input multiplied by a gain of 20V/V, you expect 80mV out. But, in a worst case situation the output could still be in saturation since we specify 100mV. So, to overcome this, you can use the REF pins to input a voltage >100mV to overcome the output range limitation. In this mode when you have 0mV input you know exactly what the output is since it is equal to the REF voltage at the REF pins. The output will increase linearly from there. 0.123V was picked since it is close to the 100mV spec meaning you can monitor a wider current range. The REF starting point can be anything over 100mV to ensure the output low range sepcification is overcome.

 
At 8:12 AM, Blogger bearchow said...

All this and yet you don't say two fundamental things about designing current sensing for wide dynamic range:

1. Lowest possible offset.
2. Largest possible shunt.

 
At 10:24 AM, Anonymous Anonymous said...

Hello,

The issue with simply focusing on the offest and shunt value limits the conversation a bit. The offset is very important, as see by the fact that the part which performed the best was the AD8217 that also has the lowest offset. But, the initial offset error can be calibrated out if necessary at room temperature. This still does not mean that it is not extremely important to current sensing. The shunt value is something that varies with each application. It is correct that a higher value shunt is better for higher resolution, but in the applications targeted by the parts mentioned, the current monitored is very large, and for power dissipation purposes it is ideal for the shunt to be as small as possible. That is why the article tries to focus on what the parts do when used in a typical mode. However, the two points are noted that offset and shunt value are very important for current sensing. Thanks for the question.

 

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