Abstract:
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.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to electrical measurement equipment and, in particular, to a range-changing circuit.  
         [0002]     It is common in electrical measurement devices to have range selecting circuits on order to provide the desired range of measurements.  
         [0003]     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.  
         [0004]     Referring to  FIG. 1 , an exemplary prior art measurement device  1  is a source measure unit (SMU) in voltage control mode (voltage sourced, current measured). An error amp A 1  controls Q OUT . A series of current sensing elements and switches (except for the lowest range) are connected in parallel and are in series with the load resistor to provide range-changing. A differential amplifier senses across R LOAD  and provides voltage feedback V FB . Feedback resistors R compare the V FB  to the V DAC  and present the corresponding error voltage to A 1 .  
         [0005]     Switches S 1  to S N  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.  
         [0006]     The switches are in series with the load; therefore an instantaneous change in R SENSE  (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&#39;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.  
       SUMMARY OF THE INVENTION  
       [0007]     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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic diagram of a prior art device.  
         [0009]      FIG. 2  is a schematic diagram of an exemplary device according to the invention.  
         [0010]      FIG. 3  is a schematic diagram of another exemplary device according to the invention.  
         [0011]      FIG. 4  is a schematic diagram of an additional exemplary device according to the invention.  
         [0012]      FIG. 5  is a schematic diagram of a further exemplary device according to the invention.  
         [0013]      FIG. 6  is a schematic diagram of another further exemplary device according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     Referring to  FIG. 2 , a measurement device  10  includes a range-changing circuit  12 . The amplifier  14  operates to apply the voltage V DAC  across the load  16  via the transistor  18 , the power supply  20  and the circuit  12 . The buffer  22  provides a buffered version of the voltage to the voltage feedback loop.  
         [0015]     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.  
         [0016]     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.  
         [0017]     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 .  
         [0018]     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 V DAC  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.  
         [0019]     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.  
         [0020]     Referring to  FIG. 3 , another range-changing circuit  12 ′ substitutes a mosfet  44  for a zener diode. The current flows through both resistors  46 ,  48  until the voltage across them reach the threshold voltage of the mosfet  44 . At that time, the mosfet  44  starts carry current in parallel with the impedance  48 . If the current is increased, the voltage drop across the impedance  46  will also increase which will cause the mosfet  44  to increase its conduction such as to reduce the voltage across the impedance  48 . The result is the total voltage across the combination of the impedances  46 ,  48  only increases a fraction of the amount that the increase of current generates across the impedance  46 .  
         [0021]     Referring to  FIG. 4 , another range-changing circuit  12 ″ adds a bias voltage source  50  to control the threshold voltage of the mosfet  44 .  
         [0022]     Referring to  FIG. 5 , another range-changing circuit  12 ′″ adds an amplifier  52 , a diode  54  and a guard impedance  56 . The three additional components act as a guard, preventing leakage from the mosfet being measured in the case of very low current measurements.  
         [0023]     Referring to  FIG. 6 , another range-changing circuit  12 ″″ adds a back-to-back mosfet  58  along with an associated bias voltage source  60  and a diode  62  to make the operation of the circuit  12 ″″ bipolar.  
         [0024]     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.  
         [0025]     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.  
         [0026]     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.