A compact and inexpensive variable resistor circuit has a value of resistance that can be set by a data signal. The variable resistor includes a number of variable individual resistances connected in series, and an electronic switch connected in parallel across each individual resistance. The values of the individual resistances are chosen to include a basic resistance value and a compensating resistance value, which corresponds to the impedance of the associated electronic switch in its closed state. The basic value is chosen from the progression 2.sup.0, 2.sup.1, 2.sup.2, 2.sup.3, and so on, so that the resistance of the variable resistor corresponds to the binary value of data applied to control the electronic switches. Other aspects of the invention include input and control circuitry which is designed to minimize its size and allow it to be packaged on an integrated circuit.
A compensation circuit includes a first comparator (12) that compares a load signal (18) generated from a resistive load (16) to a first reference voltage (V1). A second comparator (14) compares the load signal (18) to a second reference voltage (V2). The range between the reference voltages (V1 and V2) determines compensation adjustments made to the resistive load (16). A counter circuit (20) increments or decrements a thermometer code count sequence (22) according to the comparisons made by the first and second comparators (12 and 14). The thermometer code count sequence (22) controls and adjusts the resistive load (16) in order to place the load signal (18) within the range of the reference voltages (V1 and V2). The counter circuit (20) prevents the count sequence from continuously rolling over whenever the load signal (18) exceeds the range of the reference voltages (V1 and V2) by more than the maximum compensation available.
A system for simulating changes in the resistance of a living body includes an output interface circuit including a variable resistance adapted to smoothly and naturally simulate the resistance of a living body. A generator circuit includes a microprocessor under the control of software to play back digitally recorded data representative of patterns of changing resistance. The software includes a user selection feature permitting various patterns to be played back in a way that simulates changes in the resistance of living body useful for learning about the patterns.
A pseudo-random switched resistor for emulating a relatively high input impedance. The switched resistor comprises a relatively low value resistor, a semiconductor switch coupled to the resistor, and a pseudo-random pulse generator coupled to the switch for controlling the on time of the switch to emulate the relatively high input impedance. The random pulse generator comprises a plurality of cascaded circuits that are coupled to a flip flop. The plurality of cascaded circuits each comprise a shift register for receiving clock signals and for generating a sequence of (2.sup.N -1) pseudo-random numbers and a terminal count pulse at the end of every sequence, an N-bit binary counter for receiving the terminal count pulse which clocks it, and an equivalence circuit for comparing the outputs of the shift register and the binary counter and for generating an output signal when there is a match. The flip-flop generates a pulse to close the switch whenever there is a match between the output of the shift register and the binary counter as determined by the equivalence circuit.
A precise digitally-controlled variable attenuation circuit for adjusting e attenuation of a signal in an external circuit includes a signal magnitude detector, a resistance adjustment control, and a resistance divider network. The signal magnitude detector has lower and upper threshold limits representing a desired range of attenuation and is operable to receive and compare a control signal with the lower and upper threshold limits, and, in response thereto, produce either a first signal if the control signal is less than the lower threshold limit or a second signal if the control signal is greater than the upper threshold limit. The adjustment control is capable of receiving the first and second signals and is operable to produce either a digital count-down signal in response to the first signal or a digital count-up signal in response to the second signal. The resistance divider network has a fixed resistance and a digitally-adjustable device with a variable resistance. The fixed resistance is connected between a pair of terminals of an external circuit between which a signal is applied. The variable resistance is connected between one of the terminals of the external circuit and the adjustment control. The digitally-adjustable device will increase or decrease its variable resistance, and thereby correspondingly decrease or increase the attenuation by the divider network of the signal of the external circuit, in response to receiving respectively the digital count-up or count-down signal from the adjustment control.
A precision resistor using MOS devices. The present invention utilizes an NMOS resistive element to simulate the resistor. A pair of PMOS source followers are implemented to control the value of the resistor and cancel the non-linearity due to the drain-source voltage V.sub.ds. A pair of NMOS source followers serve to eliminate non-linear distortions due to the "body effect" that can exist in the resistive element. The resistor circuit of the present invention provides higher precision, linearity and high value resistors in a smaller area than prior art MOS resistors.
