JP2556923Y2 - Auto-zero circuit using ROM - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an auto-zero circuit using a ROM.

[0002]

2. Description of the Related Art A conventional example of an auto-zero circuit used in an optical time domain reflectometer will be described with reference to FIGS. First, referring to FIG. 1, reference numeral 1 denotes a receiving unit in the optical time domain reflectometer. Back-reflected laser light sent from a port of a directional coupler (not shown) is photoelectrically converted by a photodiode 2, and the photoelectrically converted current is converted into a voltage by a current-to-voltage converter 3 and then amplified by a voltage amplifier 4. , The output of the receiving unit. This output must always take a certain offset value when there is no signal other than the measurement period. Therefore, in FIG. 1, Z _s = R and Z _f =
The voltage amplifier 4 is fed back in an analog manner by an integrator 6 composed of an operational amplifier C.

[0003] Next, in FIG. 2, the output of the voltage amplifier 4 is converted from analog to digital by an analog / digital converter 7, the conversion result is processed by a CPU 8, and the digital Give feedback to.

[0004]

However, since the feedback circuit in FIG. 1 is constituted by the integrator 6, the integration time of the time constant CR is required, so that the offset cannot be executed at high speed. . Although the feedback circuit of FIG. 2 operates at high speed, it requires an analog-to-digital converter 7 and a digital-to-analog converter 9 and requires the help of the CPU of the optical time domain reflectometer itself. Has disadvantages.

The purpose of the present invention is to provide an auto-zero circuit which has solved the above-mentioned drawbacks and problems.

[0006]

SUMMARY OF THE INVENTION In an auto-zero circuit, comprising an amplifier, comprising a comparator, one input of the comparator is connected to a reference voltage source and the other input is connected to the output of the amplifier. A counter connected to the output, the counter outputs a count proportional to the output of the comparator, and a ROM that is read out using the count value of the counter as an address.
In the OM, an offset voltage to be output corresponding to the count value is described in a weighted manner. By providing a digital-to-analog converter connected between the output of the ROM and the input of the amplifier, the auto-zeroing is performed. The operation speed of the circuit was increased and the adjustment of the gain was extremely simple.

[0007]

An embodiment of the present invention will be described with reference to FIG. The feedback circuit of the present invention for the amplifier 4;
That is, the auto-zero circuit includes a comparator 10, a counter 11, a ROM 12, and a digital-to-analog converter 13, as shown.

Here, the output of the amplifier 4 which is the output of the receiving section is compared with a reference voltage in a comparator 10. This difference voltage is applied to the counter 11, and the counter 11 counts up or down until the output of the amplifier 4 matches the reference voltage and the difference voltage becomes zero. This count value is used as an address in the ROM 1 corresponding to the address.
2 is read out. By the way, ROM12
Is the offset voltage to be output corresponding to the count value. The contents of this program are described with weights, and the gain of the feedback circuit is adjusted. The digital offset voltage value read from the ROM 12 is stored in the digital / analog converter 13
Is converted to an analog offset voltage and fed back to execute auto zero.

[0009]

According to the feedback circuit of the present invention, the difference voltage between the output of the amplifier 4 which is the output of the receiving section and the reference voltage can be detected at high speed in the counter 11 depending on the clock frequency, and the detection result is obtained. Reading of the digital offset voltage value from the ROM 12 and conversion by the digital-to-analog converter 13 do not require any special time, and compared with a feedback circuit including an integrator suffering from a time constant as shown in FIG. Operating speed is remarkably high. The operation speed is equal to that of the feedback circuit shown in FIG.
Auto-zero can be performed without requiring a PU, which is advantageous in this regard. Further, the adjustment of the gain of the feedback circuit only needs to weight the contents of the program described in the ROM 12, and this is extremely simple.

[Brief description of the drawings]

FIG. 1 is a diagram showing a conventional example of an auto-zero circuit.

FIG. 2 is a diagram showing another conventional example of an auto-zero circuit.

FIG. 3 is a diagram showing an auto-zero circuit of the present invention.

[Explanation of symbols]

 4 Amplifier 10 Comparator 11 Counter 12 ROM 13 Digital / Analog Converter

Claims (1)

(57) [Scope of request for utility model registration] 1. A counter comprising an amplifier, comprising a comparator, one input of the comparator connected to a reference voltage source and the other input connected to the output of the amplifier, and a counter connected to the output of the comparator. Is a ROM that outputs a count in proportion to the output of the comparator, and executes reading using the count value of the counter as an address.
In the ROM, an offset voltage to be output corresponding to the count value is described in a weighted manner.
An auto-zero circuit using ROM comprising a digital-to-analog converter connected between the output of the OM and the input of the amplifier.