Patent Application
20060192547
The present invention relates to electrical measurement equipment and, in particular, to a range-changing circuit.
It is common in electrical measurement devices to have range selecting circuits on order to provide the desired range of measurements.
The simplest form is simply to use a selector switch to provide the desired range. More complex automatic range-changing circuit may use relays controlled by the value measured by the measurement device to select a range that puts the measured value within a desirable range. This process becomes more complicated as greater ranges, accuracies and speed are desired. Heretofore, these factors have created the need for more circuitry and complexity to produce the desired levels and performance.
Referring to
Switches S1 to SN may be electromechanical relays or solid-state switches with their attendant bootstrapping components to eliminate switch leakage. All the switches are controlled by a microprocessor in response to the voltage measured on a current sensing element selected by the microprocessor. The microprocessor thus choosing the desired range.
The switches are in series with the load; therefore an instantaneous change in RSENSE (by turning on a switch) will present a transient to the output. In the case of solid-state switches a “ramping” circuit is generally used to “fade” in the new element in parallel with the old, allowing the gain bandwidth of the loop to minimize the glitch. Because of the wide dynamic range of resistors, timing the ramps can be problematic and usually results in a tradeoff, where the higher current ranges are switched slower than they need to be, while the lower current ranges may have a larger transient. These glitches and transients are usually present at a time after the output has settled and are in response to measurements made by the A/D converter. If a different range is needed the corresponding process of selecting a range may make a transient that may take 100's of ms to settle at low currents. In many cases, noise may cause “hunting” in which a never-ending series of glitches is present.
A range-changing circuit includes an array of graduated impedances in serial relationship, and a voltage sensing and limiting switch across one of said impedances. The switch limits the voltage across said one of the impedances in response to a voltage sensed by the switch.
Referring to
The circuit 12 includes an array of graduated impedances 24, 26, 28, 30, 32, 34, for example, having impedances as labeled. Across each of the impedances, are back-to-back zener diodes 36 having, for example, a 4 volt zener voltage.
The amplifiers 38 and switches 40, 42 provide sensing pick off points of the voltages developed across the circuit 12. The switches 40 allow sensing of the voltages of the circuit 12 for measurement purposes, while the switches 42 allow feedback to a control circuit when forcing current through the load 16. It should be noted that the series nature of the circuit 12 allows sourcing and measuring be decoupled as the pick off points can be selected independently for the two purposes.
In this example, each of the impedances 24, 26, 28, 30, 32, 34 cover 2 decades of current. For example, if 1 volt is impressed on a load 16 having a value of 10 TΩ then 100 pA of current will flow and 0.4 volts will exist across the impedance 34.
Once the current exceeds 1 nA, the zener diodes across the impedance 34 will clamp and limit the voltage to 4 volts. Now an additional 4 volts may develop across the impedance 32 allowing a load current up to 100 nA. As VDAC is increased, the clamps turn on until a range remains unsaturated. At 100 mA, 20 volts would be present across the array, but only the tap on the impedance 26 would not be saturated. The voltages across the impedances are measured with respect to ground S. In this manner, it can be seen that the circuit 12 is “automatically” switching ranges without any instructions from the actual measurement device.
It should be pointed out the diodes 36 are arranged in back-to-back manner to allow bipolar measurements, otherwise a single diode for each range would be acceptable.
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The range-changing circuit of the invention changed ranges “automatically” without needing commands from a controller. One or more of the graduated impedances effectively has a voltage-sensitive switch across it that senses the voltage and limits the voltage drop for that range impedance. This simplifies and miniaturizes the circuitry. In addition, range switching is smoother and more reliable.
In the case of forcing current, it may be desirable to provide range impedance shorting to disable higher value sense elements to remove them from the array.
It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.
