JPH051956A - Temperature difference detecting circuit for calorimeter - Google Patents

Abstract

PURPOSE:To amplify a low-level signal without using a transformer by converting the DC voltage generated on a thermocouple into AC with a chopper, and setting the impedances of the thermocouple and a feedback resistor equal. CONSTITUTION:The DC output voltage of a thermocouple 1 is converted into AC voltage by a chopper 2 driven by a switch control section 8, and it is fed to a differential amplifier 4 and differentially amplified. Since the chopper 2 is used in this circuit constitution, the offset voltage of the differential amplifier 4 liable to generate an error is canceled. The offset voltage in the middle is also canceled by an AC amplifier 5. Since the impedances of the thermocoupler 1 and a feedback resistor 3 are set equal for the offset current, no offset voltage appears at the output. A temperature difference detecting circuit for a calorimeter generating no error can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a calorimeter temperature difference detection circuit, and more particularly to an isothermal control type calorimeter temperature difference detection circuit used as an optical power standard.

[0002]

2. Description of the Related Art In an optical power meter, optical power is converted into temperature (heat) and detected in a jacket that takes heat insulation into consideration. At this time, a thermocouple is used as the temperature difference detecting element. However, since the output voltage of a pair of thermocouples is extremely low, many thermocouples are connected in series. Nevertheless, the output voltage is low, on the order of nanovolts.

For amplification of such an extremely low level signal,
The DC input is AC-converted by the chopper, boosted by the transformer, received and buffered by the operational amplifier, and synchronously rectified by the chopper and returned to DC. Then, a method of receiving again with an operational amplifier is often used.

[0004]

The problem with this method is that the transformer itself is large and heavy due to the use of the transformer. In addition, it is necessary to shield the transformer in order to prevent noise from disturbance, which further increases the size and cost. The present invention, which is also a big problem, was made by paying attention to such a point, and its purpose is to
It is an object of the present invention to provide a temperature difference detection circuit for a calorimeter capable of amplifying a low level signal without using a transformer.

[0005]

DISCLOSURE OF THE INVENTION The present invention for solving the above problems is directed to a thermocouple for detecting a temperature difference, a chopper means for converting an output voltage of the thermocouple into an AC voltage, and an AC voltage generated by the chopper means. Receiving the output of the first differential amplifying means, a chopper means for synchronously rectifying the AC output voltage of the AC amplifying means and converting it into a DC voltage, and synchronizing with the chopper means. An impedance of the thermocouple and an impedance of the feedback resistance are provided, which includes a second differential amplifier that receives the rectified DC voltage and a feedback resistor that feeds back the output of the second differential amplifier to the input of the first differential amplifier. It is characterized by the fact that and are equal.

[0006]

In the present invention, the DC voltage generated in the thermocouple is converted into AC by the chopper and the AC amplifying means is used, so that the offset voltage generated in the middle is not output. Further, since the chopper is used, the offset voltage of the differential amplification means is also canceled. Further, since the impedance of the thermocouple and the impedance of the feedback resistance are equal, there is no influence of the offset current on the input side. As a result, a low level signal can be amplified without any error.

[0007]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a time chart showing the operating state of the apparatus of this embodiment.

The output of the thermocouple 1 is es [volt], and the impedance is Rs [ohm]. The DC output voltage es of the thermocouple 1 is converted into an AC voltage by the chopper 2 driven by the switch controller 8. This AC voltage is received by the differential amplifier 4 and differentially amplified by the amplification factor A1. Therefore, the output voltage e1 of the differential amplifier 4 is A1.
(Es-ef) and A1. (Es-ef) appear alternately. By passing this voltage through an AC amplifier 5 having an amplification factor A2, + A1.A2 (| es-ef |), -A1.A
An alternating voltage of 2 (| es-ef |) appears as e2.

This e2 is synchronously rectified by a chopper 6 driven by a switch control section 8 to obtain a voltage amplification factor A3.
The differential amplifier 7 receives a voltage of A1.A2.A3 (es-ef) as the output voltage eo of the differential amplifier 7. Then, this voltage is divided by the resistor 3 and fed back. Here, if A1, A2, and A3 are sufficiently large, the gain G is determined by the feedback ratio β that allows the voltage division ratio of the resistor 3.

Therefore, e0 = G · es, where G = A / (β · A) and β = Ra / (Ra + Rb) As a result, G = (Ra + Rb) / Ra (1) and thermoelectric Impedance Rs of pair 1 and impedance of feedback resistor 3 (Ra.Rb) / (Ra + R
b) and should be equal. (2) Ra and Rb are determined from these conditions (1) and (2).

In this circuit configuration, since the chopper 2 is used, the offset voltage of the differential amplifier 4 which easily causes an error is canceled. Also, by the AC amplifier 5,
The offset voltage on the way is also canceled.

Generally, an ultra low noise amplifier has a relatively large offset voltage and offset current. Of these, the offset voltage is canceled for the above reason. However, the offset current (Ioff) is generated as an offset voltage (eoff) at the output when the impedances of the thermocouple 1 and the feedback resistor 3 are different (impedance difference Rd).

The offset voltage eoff is eoff = Ioff.Rd.G.

In this embodiment, since the impedances of the thermocouple and the feedback resistance are set equal under the condition (2), the offset voltage eoff caused by Ioff is set.
Does not appear in the output.

In setting Ra and Rb as described above, after the above calculation, a resistor having a matching resistance value is mounted, a switch is selected from a plurality of resistors mounted in advance and switched, and a ceramic substrate is provided. A method of trimming the thin film (thick film) resistance by laser light or the like is conceivable.

FIG. 3 is an explanatory diagram showing the configuration of the embodiment using the above-mentioned switch switching resistor.

In this configuration, a combination of a plurality of resistors and switches (semiconductor switches, relays, etc.) is prepared as Ra and Rb of the feedback resistor 10, and a resistance value measuring circuit 11 for measuring the resistance value of the thermocouple is prepared. To do. Then, the resistance measuring circuit 11 is configured to select a switch having a resistance that matches the above conditions (1) and (2) based on the measurement result. By doing so, it is possible to realize a measurement circuit that does not have an error and that matches the characteristics of the thermocouple 1. Therefore, even if the thermocouple is replaced, it can be dealt with instantly. The resistance value is measured with a constant current source,
It can be realized by combining with an amplifier and an A / D converter.

[0019]

As described in detail above, according to the present invention, the impedance of the thermocouple serving as the signal source and the impedance of the feedback resistor are set to be equal, so that a temperature difference detecting circuit for a calorimeter that does not cause an error. Can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a measurement state of the device according to one example of the present invention.

FIG. 3 is a configuration diagram showing a configuration according to another embodiment of the present invention.

[Explanation of symbols]

 1 Thermocouple 2 Chopper 3 Feedback resistance 4 Differential amplifier 5 AC amplifier 6 Chopper 7 Differential amplifier 8 Switch control section

Claims (1)

Claims: 1. A thermocouple (1) for detecting a temperature difference, a chopper means (2) for converting an output voltage of the thermocouple (1) into an AC voltage, and a chopper means (2). The first differential amplifier means (4) for receiving the generated AC voltage, the AC amplifier means (5) for receiving the output of the first differential amplifier means (4), and the AC output voltage of the AC amplifier means (5) Chopper means (6) for synchronously rectifying and converting to DC voltage, and second differential amplifying means (7) for receiving the DC voltage synchronously rectified by the chopper means (6).
And a feedback resistor (3) for feeding back the output of the second differential amplifying means (7) to the input of the first differential amplifying means (4), and the impedance of the thermocouple (1) and the feedback resistor (3). The temperature difference detection circuit for a calorimeter is characterized in that the impedances of the two are equal.