N channel low current sensing after biasing with AD5686

Question

Hi folks, 
 I have a question regarding multichannel current sensing. First a description of the application: So far I have the AD5686R as a 16 bit, 4-channel precision voltage source (0-5V). At each channel of the voltage source I connect a 50kOhm resistor and in series to this, I connect our device that has typically a very low resistance (few ohms) and at a certain voltage switches to high resistance (few 10 kOhm). In the end, I want to sweep the voltage and measure the current for each voltage step. To depict this easy circuit for one channel: AD5686R voltage source channel ------- 50kOhm resistor ------- my device ------ GND What we are now interested in, is to sense the current flowing through the device (=the current through the 50kOhm resistor) that is typically in the order of 10&micro;A (up to a few 10&micro;A). How and with which of the plenty of different devices in the AD product line would you measure the current passing through each of the channels? Two ideas I have in mind so far: Measure the voltage drop over the 50kOhm resistor by measuring the voltage drop over my device and subtract it from the applied voltage from the AD5686R and subsequently convert it with the given 50kOhm resistance to the corresponding current value. Or by adding a small shunt resistor over which to measure a voltage drop and converting it to the corresponding current. 
 As I want to scale the whole application to N channels, I chose as voltage source the AD5686R that one can daisy chain and thus obtain N*4 channels with N AD5686R. Also, I want to communicate with the corresponding device via SPI or I2C, similar to the AD5686R and for rapid prototyping it would be great to choose a device where there exists a similar evaluation board like the one for the AD5686R such that I can quickly build up the test system by connecting a similar evaluation board with the Arduino that already controls the AD5686R on the EVAL-AD5686R. 
 I am looking forward to your ideas and suggestions! 
 Best regards, 
 Stefan

iandal · Accepted Answer

Hi Stefan, 
 Reading through your intended application and control requirements, I suggest looking at the LTC2686 (8-channel) and the LTC2688 (16-channel). The key to both of these is the integrated current sense mux out for all VDAC channels. 
 LTC2686 Datasheet and Product Info | Analog Devices LTC2688 Datasheet and Product Info | Analog Devices 
 However, I just noticed that the current you're expecting to sense is in the 10's uA. In this case, we might need a more accurate measurement than the on-chip current sense of our DACs could provide. 
 The two of your suggestions could work, and these are generally the methods of measuring current. But as your requirement is quite high, I'll have to check with our other teams focused on current sensing applications to suggest a better solution for you. I'm just curious, what is the intended application for this and could you give more details on the functional block this circuit plays on your design. We could discuss it offline if you prefer. Just send me an email. 
 ~ian

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N channel low current sensing after biasing with AD5686

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