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# Ammeter theory of operation

10/27/2010 12:15 PM EDT
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re: Ammeter theory of operation
11/24/2010 9:04:36 PM
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I guess there must be a tradeoff in terms of a given maximum current requiring a larger MOS area, and noise and leakage issues at the low end of the range. But certainly for a great many applications the device seems to strike a judicious balance, and I like the idea of "noiseless" ranging. One thing that would worry me a bit would be signal-induced self-heating at higher currents, although this could in principle be compensated for dynamically, given an accurate "plant model". Log compression with bipolars of course has a long history and a set of associated issues, including the ubiquitous temperature-compensation resistor somewhere. At least small-geometry bipolars can have very low leakage---I used some selected RF transistors as part of a pulsed-reset circuit in charge preamps that had less than 100fA collector-base leakage at room temperature and 5V bias.

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re: Ammeter theory of operation
11/3/2010 12:21:34 PM
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For the general case, you use resistors in as in Fig 1. You simply swap out the resistors that give you feedback to the summing node; these resistors are your ranges. As an example, let's assume that we have an OPAMP that has rails of +1V to -1V. If you have 6 resistors each of 1*10^(powers of 1-6), you can switch out each resistor with a relay to represent ranges of 1uA, 10uA, 100uA, 1mA, 10mA, and 100mA. If I were going to make an ammeter on a breadboard or out of discrete parts, I would make it in the design of Fig 1.

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re: Ammeter theory of operation
11/1/2010 1:42:06 AM
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This is interesting explanation on the Ammeter. Typically how can we achieve measurement in different ranges? for example, uA, mA, and A?

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re: Ammeter theory of operation
10/28/2010 1:11:49 AM
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CORRECTION: The PocketPico read down to 20pA, not 20fA. Typo on my part.

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re: Ammeter theory of operation
10/28/2010 12:35:11 AM
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The thing to keep in mind any time a thermal voltage term exists is that it has an exponential relationship on the current; however, the thermal voltage can be compensated through standard temperature compensation techniques. My preferred method is translinear loops so the temperature dependence becomes completely a function of the mobility (mean-time between collisions). The translinear loop approach can be found described in the work of Dr. Brad Minch. As far as the circuit mismatch, the threshold current is the largest issue, but this is easily addressed by common-centroid layout and burning area. A good reference for this is Kinget [1]. Subthreshold MOSFET devices have very clean physics as soon as you address the thermal voltage compensation, so in theory, one could push these devices well into the fempto-ampere range if enough area was burned for match. The PocketPico can read down to 20fA, and each device is calibrated individually. [1] Peter R. Kinget,“Device mismatch and tradeoffs in the design of analog circuits,” Solid-State Circuits, IEEE Journal of, vol. 40, no. 6, pp. 1212–1224, June 2005.

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