JPH0446412A - Variable impedance load - Google Patents

Abstract

PURPOSE:To miniaturize a variable impedance load and to reduce the cost by using a multiplication type D/A converter and an operational amplifier in combination, and connecting a fixed impedance between an input point and the output terminal of the operational amplifier. CONSTITUTION:A fixed impedance Z is connected between an input point IN and the output terminal of an operational amplifier OA3. a current output resulting from the product between a voltage input Vi and a digital control input CS is outputted from an Io terminal of a D/A converter DAC, which incorporates a feedback resistor Rf of an operational amplifier OA2. The input CS is an n-bit digital signal, normalized by taking a full scale as '1', and assuming the normalized value as K, an output voltage Vd of the D/A converter is expressed as Vd=K.Vi. Thus, the output voltage Vo of the amplifier OA3 is expressed as Vo=(2L-1)-Vi and an equivalent ground impedance is Z/(2-2K) according to a prescribed equation. Since the small sized D/A converter and the operational amplifiers integrated in an IC are inexpensive, the variable impedance load by digital control is manufactured small in size and inexpensively.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a variable impedance load used in a balanced circuit 1&M of a 2-wire to 4-wire conversion circuit in a telephone line.

[Conventional technology]

Traditionally, variable impedance loads such as balanced networks have been switched and combined using mechanical or electronic switches such as capacitors or resistors of different values.

l.

2゜ [Problem to be solved by the invention] Conventional variable impedance loads have the problem that in order to widen the variable range or reduce the variable step amplitude, a large number of circuit elements must be prepared. there were.

The present invention does not involve switching circuit elements, but by combining the same circuit element and electronic circuit means.
The purpose is to provide a single-end grounded load with isometrically variable impedance.

rays] In order to achieve the above object, in the present invention, multiplication 1j2D
-A converter and aiIyt amplifier are combined, and a fixed impedance is connected between the input point and the point where the input point voltage is amplified positively or negatively by a digital control signal, and one end is connected. ! We have obtained a type of variable impedance load.

[Saku tears]

In a load configured as above, if one terminal connection point of the fixed impedance is twice as high as the input point voltage with respect to ground, and the output impedance is zero, the equivalent impedance to ground at the input point is the fixed impedance. It is 1/(1-K) of

〔Example〕

An example will be described with reference to the drawings. In FIG. 1, a fixed impedance Z is connected to an input point IN and an operational amplification! Connected between the output terminals of 10A3. D-A converter D
AC uses the most common multiplication type in which the product of the voltage input Vi and the digital control input C8 is output as a current output from the output, and has a built-in feedback resistor Rf of the operational amplifier OA2. The control input C8 is an n-bit digital value, but it is normalized with the full scale as 1.
If this is expressed as K, the output voltage as a DA converter is expressed by the following equation (1).

Vd = -K.Vi (1) Therefore, the output voltage Vo of the operational amplifier A3 is expressed by the following equation (2).

Vo= (2 to 1) ・Vi (2)
Here, if the current flowing through the fixed impedance 2 is tIz, this is expressed by the following equation (3).

Equation (3) shows that the equivalent ground impedance seen from the input point IN is Z/(2-2K).

〔Effect of the invention〕

Since the present invention is configured as described above,
This produces the effects described below.

Since small IC-based DA converters and operational amplifiers are available at low cost, it is possible to manufacture a digitally controlled variable impedance load in a small size and at low cost.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing one embodiment of the present invention. IN ・・・・・・・・・・・・−・ Input point Z ・
・・・・・・Fixed impedance OAI, OA2
．． OA3... Operational amplifier DAC...
... D-A converter R, R/2 ...
・・・ Resistor drawing

Claims (1)

[Claims] A variable impedance load with one end grounded, which combines a multiplier type DA converter and an operational amplifier, and connects a fixed impedance between the input point and the point where the input point voltage is amplified positively or negatively by a digital control signal.