JPH04218902A - Digital variable resistor - Google Patents

Abstract

PURPOSE:To improve stability to ambient temperature by setting a desired resistance value in steps of one divided by the N-th power of 2 (N is the number of bits of digital data for resistance value setting) and by providing first, second and third switch circuits to each separate resistor. CONSTITUTION:Separate resistors Ra1 to Ra8 which form first resistance series circuit RC1 and separate resistors Rb1 to Rb8 which form second resistance series circuits RC2 are connected in series between first terminal TL and second terminal TH. A desired resistance value is set in steps of one divided by the N-th power of 2 (N is the number of bits of digital data for resistance value setting). Each of switches SWa1 to SWa8 and SWb1 to SWb8 of first and second switch circuits SWa, SWb are connected in parallel to each of separate resistors Ra1 to Ra8 and Rb1 to Rb8 which are cascade-connected. An intermediate terminal TC is connected to a selective terminal SWc0 of a third switch circuit SWc. Thereby, resistance value variation to temperature variation is reduced and temperature stability is improved.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital variable resistor whose resistance value can be set using a digital code.

0002

2. Description of the Related Art As a digital variable resistor whose resistance value can be changed by a digital code inputted from the outside, the one shown in FIG. 6 is conventionally known.

In this figure, the overall resistance value R is divided equally into 1/256 so that the resistance value can be set in 1/256 steps between the lower resistance terminal TL and the upper resistance terminal TH of this variable resistor. 255 fixed resistors r1, r2, . . . , r255, each having a resistance value R0, are connected in series.
Connection point of r2,..., r255 and upper resistance terminal TH
A switching element FET is connected between the terminal TC and the intermediate terminal TC.
(Tr0, Tr1,..., Tr255) are connected. These FETs (Tr0, Tr1,..., Tr2
55) are provided with decoders D0, D1, .
..., D255 is connected.

[0004] These decoders D0, D1,..., D2
By inputting an 8-bit digital code for setting the resistance value to 55, the FET (Tr0, Tr1,...
, Tr255) is selectively made conductive, and the resistance value between the intermediate terminal TC and the lower resistance terminal TL or between the intermediate terminal TC and the upper resistance terminal TH can be set to a desired resistance value.

Here, each fixed resistor r1, r2,...
, r255 are often constructed by silicon resistors made using thin film technology.

[0006]

[Problems to be Solved by the Invention] In the conventional digital variable resistor described above, in order to set the resistance value with an accuracy of 1/256th step using an 8-bit digital code, a large number of fixed resistors of 255 are required. r1, r2,・
..., r255 must be connected in cascade. In this way, the fixed resistors r1, r2, . . . constitute the variable resistor.
・If the number of r255 is large, when setting the resistance value,
Since the temperature characteristics of the individual resistors are added together, the temperature characteristics of the set resistance value become worse, resulting in a lack of stability with respect to ambient temperature.

The present invention was proposed in order to solve these problems, and an object of the present invention is to provide a digital variable resistor with excellent temperature stability and a small change in resistance value with respect to temperature changes.

[0008]

[Means for Solving the Problem] In order to achieve this object, the present invention connects a plurality of individual resistors in series between a first terminal and a second terminal, and provides digital data for setting a resistance value. By switching the connection position of the intermediate terminal connected between the first terminal and the second terminal by In a digital variable resistor that can set a desired resistance value between the first terminal and the intermediate terminal or between the intermediate terminal and the second terminal, the overall resistance value R between the first terminal and the second terminal is The minimum resistance value unit R0 is 1/2 to the Nth power of
Then, for this minimum resistance value unit R0, 2 to the 0th power times, 2
1 times the 1st power, 2 times the 2nd power, 2 times the 3rd power, ..., N- of 2
A first resistor series circuit consisting of N individual resistors multiplied to the first power and connected in series, and a second resistor series circuit consisting of N individual resistors having the same configuration as the first resistor series circuit. a resistor series circuit connected in series between a first terminal and a second terminal, and connected in parallel to each individual resistor constituting the first resistor series circuit. a first switch circuit having a first terminal and a second switch circuit having a plurality of switches connected in parallel to each individual resistor constituting a second resistor series circuit; A third switch circuit is provided to switch the connection position of the intermediate terminal connected between the resistor series circuits, and the third switch circuit is switched according to the resistance value setting data to switch the desired position in the first resistor series circuit. When an individual resistor is selected, the first switch circuit is controlled so that only the switch connected in parallel to the selected individual resistor is turned off, and the first switch circuit is controlled so that only the switch connected in parallel to the selected individual resistor is turned off. 2
between the first terminal and the intermediate terminal or between the intermediate terminal and the second terminal by controlling the second switch circuit so that only the switches connected in parallel to the individual resistors in the resistor series circuit are turned on. It is characterized by setting a desired resistance value to .

[0009]

[Operation] According to the above-described configuration, when one of the plurality of switches constituting the first switch circuit is turned off, the switch of the second switch circuit corresponding to this switch is turned on. Since the resistance value between the first terminal and the intermediate terminal or between the intermediate terminal and the second terminal is controlled by the resistance value setting data while keeping the overall resistance value between the first terminal and the second terminal at R, The desired resistance value can be set.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the digital variable resistor according to the present invention will be described in detail below with reference to the drawings. FIG. 1 shows the circuit configuration of this digital variable resistor.

In this figure, there is a temperature difference between the lower resistance terminal TL, which is the first terminal, and the upper resistance terminal TH, which is the second terminal, forming a first resistance series circuit RC1 consisting of 16 resistors in total. Compensated individual resistors Ra1 to Ra8 and temperature compensated individual resistors R forming a second resistor series circuit RC2
b1 to Rb8 are connected in series. These individual resistors Ra1 to Ra8 and Rb1 to Rb8 have both terminals TL,
Let the total resistance value between TH be R, and 25 of this total resistance value R
If 1/6 is the minimum unit resistance value R0, then R0 x 20
, R0×21, R0×22, R0×23,..., R0
The resistance values are sequentially x27. For example, if the overall resistance value R is 10 kΩ, the resistance value R0 of the individual resistors Ra1 and Rb1 is 39Ω, and the resistance value R0 of the individual resistors Ra8 and Rb8 is 5 kΩ. Here, each fixed resistor Ra1 to R
a8 and Rb1 to Rb8 are constructed of silicon resistors made by thin film technology or chip resistors.

Each switch S of the analog switch sections SWa and SWb constituting the first and second switch circuits
Wa1 to SWa8 and SWb1 to SWb8 are respectively connected in parallel to each of the cascade-connected individual resistors Ra1 to Ra8 and Rb1 to Rb8. Further, the intermediate terminal TC is connected to the selection terminal SWc0 of the analog switch section SWc, which is the third switch circuit, and the selected terminals SWc1 to SWc8 of this switch are connected to the connection points of the adjacent individual resistors Ra1 to Ra8 and Rb1, respectively. be done.

The output terminal of the 8-bit shift register IC2 controlled by the control logic circuit IC1 is connected to the input terminal of the encoder IC3 and the analog switch section S.
It is connected to the control terminal of Wb, and the knot circuit IC
4 to the control terminal of the analog switch section SWa. Further, the output terminal of the encoder IC3 is connected to the control terminal of the analog switch section SWc. Serial data for resistance value setting is input to the shift register IC2 from the input terminal IN1. This shift register IC2
Serial data used when connecting variable resistors in cascade is output from an output terminal OUT1 connected to the output terminal OUT1. Reference symbols IN2, IN3, and IN4 are a clock signal input terminal, a chip selection signal input terminal, and a strobe signal input terminal connected to the control logic circuit IC1.

In the digital variable resistor configured as described above, serial data input from the setting data input terminal IN1 is converted into 8-bit parallel data by the shift register IC2, and the most significant of this parallel data is converted by the encoder IC3. When the “H” bit is detected,
The analog switch section SWc is switched based on the detection output from the encoder IC3. Further, the 8-bit parallel data output from the shift register IC2 is inverted by the NOT circuit IC4, and each bit data of this inverted parallel data is used for each switch S of the analog switch section SWa that corresponds one-to-one to each bit data.
Wa1 to SWa8 are turned on and off, respectively. In addition, each switch SWb1 to SWb of the analog switch section SWb
8 are respectively turned on and off by 8-bit parallel data output from the shift register IC2.

Here, setting data input terminals IN1 to 1
For example, if "32" is input in decimal notation, parallel data of "00100000" is output from the shift register IC2. In this setting data, the most significant "H" bit is located in the third from the highest order, and the resistor Ra
Since the 8 side corresponds to the upper bit, the analog switch section SWc is switched so that the selection terminal SWc0 is connected to the selected terminal SWc6.

Further, the inverted parallel data that has passed through the NOT circuit IC4 is "11011111", and if the right side is arranged in accordance with the arrangement of the analog switch section SWa corresponding to the upper bit, it becomes (11111011). This inverted parallel data causes switches SWa1 to SWa to
8 is turned on and off, so in this example, switch SWa
6 is off and the remaining switches are on.

When the parallel data from the shift register IC2 that controls the analog switch section SWb is rearranged so that the upper bit is on the right side, it becomes (00000100), so in this example, only the switch SWb6 is on.
The other remaining switches are turned off.

FIG. 2 is a diagram equivalently showing the connection state in this case. In this way, in this digital variable resistor, the resistance values of the terminal TL and the intermediate terminal TC can be changed by the digital code input to the input terminal IN1 without changing the overall resistance value R between the terminals TL and TH. In this digital variable resistor, a small number of fixed resistors (eight in practice) are used to set the resistance value, so the temperature characteristics of a large number of fixed resistors do not appear as an addition. Good temperature stability.

FIG. 3 shows an example in which the digital variable resistor described above is used as the variable resistor VR1 for adjusting the offset and the variable resistor VR2 for adjusting the gain of the operational amplifier OP1.

The output in this example is VO-VI((R2/R1)+(R×
setting value)/(R1×255)).

FIG. 4 shows an example of use as an 8-bit A/D converter, in which a reference voltage is connected to a variable resistor VR3 whose resistance value is set by an 8-bit digital code. The reference voltage divided by is input to the comparator CP1, and is compared with the analog input voltage Va. Thereby, data Vd that can be output to the CPU etc. can be taken out from the output terminal.

FIG. 5 shows an example in which a variable resistor VR4 is used as an 8-bit D/A converter. In this example, a digital code V is used to set the resistance value of variable resistor VR4.
When d is input, an analog output Va is obtained from the output terminal of the comparator CP2.

[0023]

As explained above, according to the present invention, when the number of bits of the digital data for resistance value setting is N, in setting the resistance value in one step of 2 to the N power,
It is sufficient to prepare 2N individual resistors, and the number of individual resistors can be significantly reduced compared to the conventional one. In addition, the number of individual resistors actually used is N, so the temperature characteristics of the individual resistors do not add up and greatly affect the set resistance value, and it is extremely stable against changes in ambient temperature. A stable and highly accurate digital variable resistor can be constructed.

Therefore, the digital variable resistor according to the present invention is suitable for setting the reference voltage of a comparator with high accuracy using a logic signal from a microcomputer interface, and is suitable for other highly sensitive electric circuits, and has excellent temperature stability. It is possible to realize an electronic device that is

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a digital variable resistor according to the present invention.

FIG. 2 is an equivalent circuit diagram of this digital variable resistor in an operating state.

FIG. 3 is a circuit diagram showing an example in which the digital variable resistor is applied to an amplifier.

FIG. 4 is a circuit diagram showing an example in which the digital variable resistor is applied to an A/D converter.

FIG. 5 is a circuit diagram showing an example in which the digital variable resistor is applied to a D/A converter.

FIG. 6 is a circuit diagram showing an example of a conventional digital variable resistor.

[Explanation of symbols]

Ra1 to Ra8, Rb1 to Rb8...Individual resistor SWa1 to SWa8, SWb1 to SWb8, SWc
... Analog switch TL ... Lower resistance terminal TH.
... Upper resistance terminal TC ... Intermediate terminal IC1 ... Control logic circuit IC2 ... Shift register IC3 ... Encoder IC4 ... Not circuit

Claims (1)

[Claims] Claim 1: A plurality of individual resistors are connected in series between a first terminal and a second terminal, and the resistors are connected between the first terminal and the second terminal according to digital data for setting a resistance value. By switching the connection position of the intermediate terminal that is connected to the
In a digital variable resistor that can set a desired resistance value between a first terminal and an intermediate terminal or between an intermediate terminal and a second terminal with an accuracy of one step of multiplication, the first terminal and the second The minimum resistance value unit R0 is 1/2N of the overall resistance value R between the terminals, and the minimum resistance value unit R0 is 2 to the 0th power, 2 to the 1st power, 2 to the 2nd power, A first resistor series circuit consisting of N individual resistors whose numbers are 2 to the 3rd power, ..., 2 to the N-1 power, and these individual resistors are connected in series;
A resistor series circuit and a second resistor series circuit consisting of N individual resistors having the same configuration are connected in series between the first terminal and the second terminal, and the first resistor series circuit is connected in series between the first terminal and the second terminal. A first switch circuit having a plurality of switches each connected in parallel to each of the individual resistors constituting the circuit, and a plurality of switches each connected in parallel to each individual resistor constituting the second resistor series circuit. a second switch circuit having a switch, and a third switch circuit that switches the connection position of an intermediate terminal connected between the first terminal and the first resistor series circuit, and The first switch is configured such that when a desired individual resistor in the first resistor series circuit is selected by switching the third switch circuit, only the switch connected in parallel to the selected individual resistor is turned off. controlling the circuit, and controlling the second switch circuit so that only the switch connected in parallel to the individual resistor in the second resistor series circuit that corresponds one-to-one to the selected individual resistor is turned on; , a digital variable resistor characterized in that a desired resistance value is set between a first terminal and an intermediate terminal or between an intermediate terminal and a second terminal.