Let's assume I have a significant resistance between FORCE and the DUT.
SENSE is, of course, connected at the DUT.
In my case the difference may be 1.5V. Is this ok?
The whole idea of force and sense is to get the very high current, say an amp or more, to flow from force through
100-500 milliohms of trace resistance to the DUT; But I want to control the voltage at the DUT, so I run a wire from
sense over to the DUT. The input current for the sense is less than 12.5 nA. So even if you have 1kohm of
wire/trace/etc, that is microamps. How can you get 1.5V??
Standard control theory: The gain block will drive the pass transistor to try to get the voltage on the sense line
up to the desired voltage.
maybe I described it wit too less detail.
I have about 1.5 Ohms between the 5560-FOCRCE and the DUT
(the point where I want the voltage to be constant).
5560-SENSE is, of course, connected to the DUT.
At ~1A flowing I'll get 1.5V between the 5560's FORCE and SENSE pins.
My question basically is whether there is an internal maximum limit.
Check. I think it's more a matter of headroom; Force keeps going higher and higher until it can't
go any more.
I sent a private email to the product line apps engineer in case he missed this thread.
Change your OSD threshold settings.
Just to clarify, Is the resistor you are using between the FORCE and SENSE PIN an ext. sense resistor for the Current sense? Usually the recommended voltage drop between the external sense resistor should only be around .6 to .7 volts. This can be seen in the AD5560 Datasheet Rev E. Page 6 under Measure Current.
let’s take a look at the block diagram on page 4.
There is a TLine-like symbol between the FORCE pin and DUT.
Now, for clarification, let’s assume this has just a relatively high
DC-resistance, so that the voltage across it may achieve 1V-2V or so.
My question is about the maximum of this voltage.
Is this limited by headroom or by other things?
The OSD DAC setting is 0.62 volts by default. You can change it up to 5 volts.
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There are two “headrooms” to be concerned with.
1. OSD DAC (0.62 volts by default. This is usually not enough for high current loads).
2. Power supplies (2.75 volts recommended)
Ideally, there shouldn't be any significant resistance between the Force and Sense. If in any case this cannot be avoided, then this should be kept at a minimum since this would effectively decrease the maximum voltage you can achieve on your DUT side. This would be now equal to (AVDD-2.75-Vdrop of "DC-resistance" or AVSS+2.75+Vdrop of "DC-resistance" ).
Another thing to consider is the OSD circuitry which protects the DUT from overvoltages(This is explained further in the AD5560 Datasheet, page 29). You should increase the OSD DAC(higher than the expected Vdrop of "DC-resistance") as jChakuroff suggested to keep the AD5560 in normal operation under these conditions.
You mentioned earlier that there would be a flow of 1A coming from the Force, this is not possible since the maximum current range coming out of the FORCE pin is at 25mA. For a 1A current range, you should be using the EXTFORCE1. This is why I was asking if the "DC-resistance" was an external current sense resistor as it is required in this type of high current application. Can I ask what is your end application for the AD5560? This may give me additional insight on what you plan to do and a give better recommendation.
thanks for updating.
Ok, the 1A doesn’t come from the FORCE pin directly, it is flowing from the node where the FORCE pin
is connected, to the DUT (according to the block diagram in the data sheet).
I talked about relatively high resistance on the way to the DUT,
but in reality we have a series diode plus a couple of hundred mV of resistance
(for protection and other reasons), which may end up in the 1V area.
So, using the OSD DAC accordingly is the way to go.
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