JPS6246331Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a variable gain amplifier, and more specifically, it has a filter circuit for noise removal and adjusts the gain according to a plurality of input signals applied in a predetermined time series. In a variable gain amplifier configured to switch the gain, the filter circuit is disconnected when switching the gain, thereby preventing a delay in response due to the influence of the filter circuit.

For example, in a gas chromatograph, measurement signals corresponding to a plurality of measurement components are sent out in a predetermined time series. Since the magnitude of these measurement signals is widely distributed (10 ⁶ to 10 ⁷ ), it is common to use a variable gain amplifier configured to switch the gain depending on the measurement signal for signal processing. It is true. By the way, in such an amplifier,
A filter circuit is provided to remove noise received from inside and outside the device, especially at high amplification levels.
However, since such a filter circuit has a relatively large time constant, the response is delayed when switching the gain.

The present invention solves these drawbacks and will be described in detail below with reference to the drawings.

Figure 1 is a circuit diagram showing one embodiment of the present invention;
The figure is an equivalent circuit diagram of FIG. 1. In the drawing,
Ti is an input terminal, SC is an adder circuit, OP is an operational amplifier, DAC ₁ to _{DAC 3} are D/A converters, C is a capacitor, SW is a switch, CMP is a comparator, μP is a calculator, and R ₁ to _{R 3} are The resistor, To, is the output terminal.

One input terminal of the adder circuit SC has an input terminal Ti.
is connected, the other input terminal is connected to one output terminal of the calculator μP via the second D/A converter DAC ₂ , and the output terminal is connected to the inverting input terminal (-) of the operational amplifier OP. has been done. These calculators μ
The system consisting of P→second D/A converter _DAC2 →summing circuit SC constitutes an auto-zero correction circuit that corrects baseline fluctuations of the measurement system and offset voltage of the operational amplifier OP. operational amplifier
The non-inverting input terminal (+) of OP is connected to a common potential point, and the first
A D/A converter DAC ₁ is connected thereto, and a capacitor C is also connected via a switch SW. That is, one end a of the capacitor C is connected to the output terminal of the operational amplifier OP, and the other end b is connected to the switch.
Connected to the movable contact a of the SW. switch
The SW is, for example, a semiconductor switch configured as a changeover switch, with one fixed contact b
is connected to the inverting input terminal (-) of the operational amplifier OP, and the other fixed contact c is connected to the common potential point. The switching operation of the switch SW is controlled according to a program stored in the computer μP, for example. Note that the first D/A converter
As DAC ₁ , for example, a multiplication type is used. This first D/A converter DAC ₁ is also controlled according to a program stored in the computer μP, for example, and changes the feedback amount of the operational amplifier OP in stages. Here, the first D/A
The converter DAC ₁ functions as a feedback circuit that changes the amount of feedback in steps, and the computer μP functions as a control circuit that drives the first D/A converter DAC ₁ and the switch SW. This causes operational amplifier OP to operate as a variable gain amplifier. The output terminal of the operational amplifier OP is connected to one input terminal of the comparator CMP, and the other output terminal of the calculator μP is connected to the other input terminal via the third D/A converter _DAC3 . ing. The comparator CMP and the third D/A converter DAC ₃ constitute a successive approximation type A/D converter.

The operation of the circuit configured in this manner will be explained using the equivalent circuit diagram shown in FIG.

When the movable contact a of the switch SW is connected to the fixed contact b side, the capacitor C has a resistance R ₂
is connected in parallel with the circuit and acts as a low-pass filter circuit. This makes it possible to remove high-frequency noise components superimposed on the input signal V _IN .
By the way, in such a state, the variable gain amplifier will have a large time constant _CR2 , resulting in a slow response. Therefore, when changing the gain or suddenly changing the input signal V _IN by auto-zero correction, etc., switch the movable contact a to the fixed contact c side according to the program stored in the computer μP. Connecting. As a result, capacitor C
is rapidly charged to a steady state value. After the capacitor C is charged to a steady value, the switch SW is again connected to the fixed contact b side. This results in
Response delay due to the influence of time constant CR ₂ can be prevented, and responsiveness can be improved.

In particular, in the case of gas chromatographs, gain calibration is performed by applying a standard measurement signal at each measurement cycle, and auto-zero correction is performed at fixed time intervals, but by using the amplifier according to the present invention, these steps can be performed. It can save time.

As explained above, according to the present invention, in a variable gain amplifier having a filter circuit for noise removal and configured to switch the gain according to a plurality of input signals applied in a predetermined time series. , it is possible to prevent delay in response due to the filter circuit when switching the gain, and it is suitable for signal processing circuits of gas chromatographs and the like.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention;
The figure is an equivalent circuit diagram of FIG. 1. Ti...Input terminal, SC...Addition circuit, OP...Operation amplifier, DAC ₁ to DAC ₃ ...D/A converter, C
... Capacitor, SW ... Switch, CPM ... Comparator, μP ... Calculator, R ₁ to _{R 3} ... Resistor, To ...
...Output terminal.

Claims (1)

[Claim for Utility Model Registration] An operational amplifier whose non-inverting input terminal is connected to a common potential point, and an operational amplifier connected between the inverting input terminal and the output terminal of the operational amplifier, and by changing the amount of feedback in stages. a feedback circuit that switches the gain; one end is connected to the output terminal of the operational amplifier, the other end is selectively connected to the inverting input terminal or common potential point of the operational amplifier via a changeover switch, and when the inverting input terminal is connected, the feedback circuit It comprises a capacitor connected in parallel with the circuit to form a filter circuit, and a control circuit that drives these feedback circuits and a changeover switch, and simultaneously drives the changeover switch when switching the feedback circuit, and controls the other end of the capacitor. A variable gain amplifier characterized in that the capacitor is connected to a common potential point until it is charged to a steady-state value.