AN ARRANGEMENT FOR COMPENSATING FOR TEMPERATURE DEPENDENT VARIATIONS IN SURFACE RESISTANCE OF A RESISTOR ON A CHIP

Abstract: To compensate for temperature dependent variations and process variations in surface resistance of a main resistor (R1) on a chip (1), one or more compensating resistors (R11, R12 ... R1n) can be connected in series with the first resistor (R1) via normally open switches (SR11, SR12 ... SR1n). The switches are closed to connect one or more of the compensating resistors (R11, R12 ... R1n) in series with the main resistor (R1) in response to whether the voltage across resistors (R21, R22 ... R2n) produced on the chip (1) in the same process and proportional to the compensating resistors (R11, R12 ... R1n) is higher or lower than a fixed reference voltage (VR3).

Description: AN ARRANGEMENT FOR COMPENSATING FOR TEMPERATURE DEPENDENT VARIATIONS IN SURFACE RESISTANCE OF A RESISTOR ON A CHIP TECHNICAL FIELD The invention relates generally to resistors and more specifically to an arrangement for compensating for temperature dependent variations and process variations in surface resistance of resistors on a chip. BACKGROUND OF THE INVENTION When filters are produced on silicon chips, there are a number of factors that influence the transfer function of the filters. Since it is the RC-constant that sets the cut-off frequency of a filter, one can look at what causes the R, i.e. the resistance, and the C, i.e. the capacitance, to change. The surface resistance of a resistor varies with temperature. Moreover, the surface resistance can vary in response to variations in the production process. The width of the resistor on the chip can e.g. vary. In total, the resistance value can vary more than ± 50%. The capacitance value varies merely marginally and does not need compensation in the same extent. SUMMARY OF THE INVENTION The object of the invention is to provide an arrangement for compensating for such temperature dependent variations and process variations in surface resistance of a resistor on a chip. This is attained in accordance with the invention by automatically connecting one or more compensating resistors in series with a main resistor. BRIEF DESCRIPTION OF THE DRAWING The invention will be described more in detail below with reference to the appended drawing on which the single figure is a schematic illustration of an embodiment of a compensating arrangement for a resistor on a chip in accordance with the invention. DESCRIPTION OF THE INVENTION On the drawing, a main resistor Rl on a chip 1 is shown. The resistor Rl can constitute part of a filter circuit (not shown). In accordance with the invention, to compensate for temperature dependent variations and process variations in surface resistance of the main resistor Rl, the resistor Rl can be connected in series with one or more compensating resistors Rl 1, R12 ... Rln. The main resistor Rl in series with any of the compensating resistors Rl l, R12 ... Rln is connected between two terminals Nl and N2 on the chip 1. To determine whether or not the resistor Rl has to be connected in series with any of the compensating resistors Rl 1, R12 ... Rln between the terminals Nl and N2 to compensate for temperature dependent variations and process variations, a resistor R2 proportional to the resistor Rl, is connected in series with resistors R21, R22 ... R2n proportional to the compensating resistors Rl 1, R12 ... Rln between a ground terminal and a current generator 2. The resistors R2, R21, R22 ... R2n are produced on the chip 1 in the same process as the resistors Rl, Rl l, R12 ... Rln. External to the chip 1, a precision resistor R3 with low temperature coefficient is connected between the ground terminal and a current generator 3. The current generator 3 generates a reference current I through the resistor R3. In accordance with the invention, the current generator 2 generates a current I, that is identical to the reference current I generated by the current generator 3, through the resistor R2 in series with the resistors R21, R22 ... R2n. Instead of having two separate current generators 2 and 3, the reference current I through the resistor R3 can be mirrored by means of a current mirror (not shown) to flow through the resistor R2 in series with the resistors R21, R22 ... R2n. In accordance with the invention, the reference current I from the current generator 3 generates a fixed reference voltage VR3 across the external resistor R3. The current I from the current generator 2 generates a voltage VR2 across the resistor R2, and voltages VR21, VR22 ... VR2n across the respective resistor R21, R22 ... R2n. The main resistor Rl is connectable to the terminal N2 either directly via a switch SRI or indirectly in series with one or more of the compensating resistors Rl 1, R12 ... Rln via switches SR11, SR12 ... SRln, respectively. The switches SRI, SRI 1, SR12 ... SRln are e.g. transistors controlled by output signals from respective comparators Kl, Kl 1, K12 ... Kin. One input of the comparators Kl, Kl 1, K12 ... Kin is connected to the interconnection point between the current generator 3 and the resistor R3, and is thus supplied with the fixed reference voltage VR3. The other input of the comparators Kl, Kl 1, K12 ... Kin is connected to the respective interconnection point between the resistors R2, R21, R22 ... R2n, and is thus supplied with the respective voltage VR2, VR21, VR22 ... VR2n. Thus, the comparator Kl compares the voltage VR2 across the resistor R2 with the fixed reference voltage VR3 across the resistor R3. If the voltage VR2 is higher than the fixed reference voltage VR3, indicating that the resistance of the main resistor Rl does not have to be compensated for, the comparator Kl outputs an output signal to close the switch SRI to, hereby, connect the main resistor Rl directly to the terminal N2. If e.g. the comparator K12 detects that the voltage across the resistor R2 in series with the resistors R21 and R22, i.e. the voltage VR2 + VR21 + VR22, is higher than the fixed reference voltage VR3, the comparator K12 will output an output signal to close the switch SRI 2 to connect the resistors Rl 1 and R12 in series with the main resistor Rl to the terminal N2 to compensate for a variation of the surface resistance of the main resistor Rl . In this manner, one or more of the compensating resistors Rl 1, R12...Rln can be connected in series with the main resistor Rl to the terminal N2 to compensate for temperature dependent variations and process variations in surface resistance of the main resistor Rl on the chip 1.

Claims: CLAIM An arrangement for compensating for temperature dependent variations and process variations in surface resistance of a first resistor (Rl) on a chip (1), characterized in - that said first resistor (Rl) is connectable between a first terminal (Nl) and a second terminal (N2) directly via a normally open first switch (SRI) and indirectly in series with at least one compensating second resistor (Rl 1, R12 ... Rln) on the chip (1) via a normally open second switch (SRI 1, SR12 ... SRln), - that a first comparator (Kl) is adapted to compare a reference voltage (VR3), generated by a reference current (I) across a precision resistor (R3) external to the chip (1), with a first voltage (VR2) generated by a current identical to the reference current (I) across a third resistor (R2) on the chip (1), proportional to said first resistor (Rl), and generate an output signal to close said normally open first switch (SRI) to connect said first resistor (Rl) directly to said second terminal (N2) if the reference voltage (VR3) is lower than said first voltage (VR2), and - that at least one second comparator (Kl 1, K12 ... Kin) is adapted to compare the fixed reference voltage (VR3) with a second voltage generated by said current identical to the reference current (I) across the third resistor (R2) in series with at least one fourth resistor (R21, R22 ... R2n) on the chip (1), proportional to said at least one compensating second resistor (Rl 1, R12 ... Rln), and generate an output signal to close said normally open second switch (SRI 1, SR12 ... SRln) to connect said first resistor (Rl) in series with said at least one compensating second resistor (Rl 1, R12 ... Rln) to said second terminal (N2) if the reference voltage (VR3) is lower than the voltage across the third resistor (R2) in series with said at least one fourth resistor (R21, R22 ... R2n).

Inventors: OLSON ALLAN, OLSON, ALLAN
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2001
Main classification: H01C7
IPC codes: H01C7