Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1998-04-10
2001-07-17
Hofsass, Jeffery A. (Department: 2736)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S463000, C340S464000, C340S635000, C324S162000, C324S207170
Reexamination Certificate
active
06262670
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to the field of source measure units and specifically to a sample-and-hold feedback circuit for eliminating transients. A source measure unit (SMU) is an electronic instrument that is capable of providing a constant voltage (“sourcing”) while measuring current and providing a constant current while measuring voltage. Features that are desirable in this type of instrument are high resolution and dynamic range in both sourcing and measuring, fast operation, and the absence of unexpected output transients. In order to simultaneously provide high resolution and large dynamic range, it is necessary to equip the instrument with multiple ranges for both voltage and current. The changing of these ranges during testing, however, can cause unwanted transients on the output of the instrument, especially when done quickly. This effect is particularly acute when the static output level is non-zero.
The most common consequence of these transient glitches is a temporary shift of the operating characteristics of the device-under-test. For example, the SMU applies a voltage to the gate-source junction of a MOSFET transistor while another instrument is measuring the drain-source resistance of the device. If the SMU produced an output transient during a range change operation, the energy of that transient would cause the highly capacitive gate-source junction to shift its voltage. This could substantially change the drain-source resistance value of the transistor even though the DC voltage level on the gate-source junction has not changed. Consequently, data taken immediately after the transient on the resistance versus voltage relationship would be in error. Moreover, if the SMU was configured to have a very low current compliance, that is, current drive capability, the time duration of the error could be substantial, as the gate-source capacitance would be discharged to its previous voltage level at a gradual rate. A less common, but more serious, consequence also appears in MOSFET testing. If the gate-source junction is being operated at a voltage that is close to its maximum rated limit, a transient spike could produce a voltage large enough to permanently damage the sensitive gate oxide material of the device.
FIG. 1
illustrates a simple amplifier system, which is analogous of prior art SMU design topology. Assuming that the setpoint signal from the D/A converter is 1V, and the values of R1 and R2 are equal, that is, R1/R2=1, this configuration will yield an output voltage of 2V. Now consider the situation where the ratio of R1/R2 changes to 9, but the desired output level is the same 2V. In order for this to be true, the setpoint signal must decrease to 0.2V. In fact, it must decrease at exactly the same rate and at exactly the same time that the resistor ratio increases in order for the output to show no movement. In practice, however, these operations are typically executed by a microprocessor in serial fashion. That is, one signal changes, followed by the other. Even if there is very little time between the operations, any perturbation in the difference between the signals will result in significant output movement due to the high gain of the amplifier. In theory it might be possible to design analog circuitry that equates the rate of change of the two elements. In practice, however, this would be very difficult to implement, especially over a large number of ranges.
To circumvent this issue, previous designs have implemented an algorithm in which the setpoint is set to zero prior to the switching of the feedback elements. Once the new feedback in is place, the setpoint is reprogrammed for the proper output. This scheme does succeed in lowering glitching, because the output is less sensitive to changes in the feedback elements when little or no voltage is across them. There are some applications, however, where the unexpected movement of the output to zero during testing is not acceptable. It would be preferable for the output to truly maintain its programmed level during the range change process.
BRIEF SUMMARY OF THE INVENTION
The invention provides a source measure unit including an output stage amplifier having an output and an input. A primary feedback loop is connected from the amplifier output to the amplifier input. A secondary feedback loop is connected from the amplifier output to the amplifier input. A sample-and-hold circuit is connected in the secondary feedback loop to sample and store a voltage representing an output of the amplifier. A first switch is connected between the primary feedback loop and the input, and a second switch is connected between the secondary feedback loop and the input.
According to one aspect of the invention, the source measure unit includes an output stage amplifier having an output and an input. A voltage programmer has a voltage setpoint output, and a voltage program inverter is connected to invert the voltage programmer output to provide an inverted voltage setpoint. A current programmer has a current setpoint output, and a current program inverter is connected to invert the current programmer output to provide an inverted current setpoint. A current range control resistance is connected to the output stage amplifier output and is variable for selecting a range of output currents. A primary current feedback loop from the output stage amplifier output to the output stage amplifier input includes a primary current differential amplifier connected across the current range control resistance, the output of the primary current differential amplifier being summed with the current setpoint to provide a positive current control signal and with the inverted current setpoint to provide a negative current control signal. A voltage range control resistor divider is connected to the output stage amplifier output and has variable attenuation for selecting a range of output voltages. A primary voltage feedback loop from the output stage amplifier output to the output stage amplifier input includes a primary voltage differential amplifier connected across the voltage range control resistor divider, the output of the primary voltage differential amplifier being summed with the voltage setpoint to provide a positive voltage control signal and with the inverted voltage setpoint to provide a negative voltage control signal. A switching control is connected to the output stage amplifier input and has a positive current input, a negative current input, a positive voltage input, and a negative voltage input, the switching control selectively connecting one of the switching control inputs to the output stage amplifier input. Primary input switches are provided for connecting the respective control signals to the respective switching control inputs such that the current control signal is connected to the positive current input, the inverted current control signal is connected to the negative current input, the voltage control signal is connected to the positive voltage input, and the inverted voltage control signal is connected to the negative voltage input.
A secondary current feedback loop from the output stage amplifier output includes a secondary current differential amplifier connected across a secondary current resistance at the amplifier output. A secondary voltage feedback loop from the output stage amplifier output includes a secondary voltage differential amplifier connected across a secondary voltage resistor divider at the amplifier output. A sample-and-hold circuit has an input connected to the outputs of the secondary differential amplifiers and has a secondary output and an inverted secondary output. A sampling input switch is provided for connecting the secondary current amplifier to the sample-and-hold circuit input. A voltage sampling input switch is provided for connecting the secondary voltage amplifier to the sample-and-hold circuit input. Secondary input switches are provided for connecting either of the secondary feedback loops to the respective switching control inputs such tha
Goins Davetta W.
Hofsass Jeffery A.
Pearne & Gordon LLP
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