JPH0439825B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to an analog switch drive device that connects a plurality of analog switches in parallel to the same power source and selectively controls conduction. [Prior Art] An analog switch drive device used in an image sensor such as a facsimile machine will be described below. As a conventional device of this kind, for example, there is one that uses an image sensor to read the color shading of an area divided into N areas, and sequentially outputs an amount of electricity proportional to each shading to one terminal. FIG. 1 shows a configuration diagram of a conventional device, and the analog switch equivalent circuit in the diagram is as shown in FIG. In FIG. 2, an N-channel transistor 31 and a P-channel transistor 32 are configured in a combination. When a positive voltage is applied from an analog switch control terminal 27, for example, a positive voltage is applied to the gate of the N-channel transistor 31. At the same time, the positive voltage applied from the analog switch terminal 27 is inverted by the inverter 30, and a negative voltage is applied from the output of the inverter 30 to the gate of the P-channel transistor 32. In this case, the N-channel transistor 31 is at a positive voltage, the P-channel transistor 32 is at a negative voltage, and the source and drain of the transistor and the input/output terminals 28 and 29 of the analog switch are conducted, and the signal from the analog switch terminal 28 is connected. is sent to analog switch terminal 29, and the signal from analog switch terminal 29 is sent to analog switch terminal 2.
8. A circuit that functions like this is an analog switch. In the circuit of FIG. 2, for example, considering the analog switch 1 of FIG. 1, the analog switch control terminal 27 of FIG. 2 is connected to the terminal 18 of the analog switch 1 of FIG. teeth,
This is equivalent to point P, which is the intersection of analog switches 1 and 2 and photoelectric conversion circuit 13. Similarly, the analog switch terminal 29 in FIG. 2 corresponds to the intersection Q between the analog switch 1 and the DC amplifier circuit 12. In FIG. 1, the photoelectric conversion circuit 13 is equivalently a series circuit of a photodiode 16 and a capacitor 17, and the charge accumulated in the capacitor 17 is passed through the diode 16 depending on the strength or weakness of the light, and a reverse current is generated. Flow and discharge. Therefore, by looking at the amount of charge in the capacitor 17, the intensity of the light can be determined.
This is a circuit that can judge the weakness. In FIG. 1, analog switches 1 and 2 and N circuits 25 and 26 equivalent to the signal detection circuit 24 constituted by the photoelectric conversion circuit 13 are lined up, and the output points of these signal detection circuits 24, 25, and 26 are lined up. There are control analog switches 8 and 33 for controlling Q reset and light amount detection time, and a DC amplifier circuit 12 for extracting signals from each signal detection circuit. Next, the operation will be explained. The N signal detection circuits 24, 25, and 26 each detect an amount of electricity depending on the color shading (white/black) of the corresponding area. First, in the signal detection circuit 24, if the terminal 18 is "L", the terminal 19 is "H", and the terminal 34 is "H" at time _t0 , as shown in FIG. ,33 is ON
Therefore, the positive voltage of the power supply from terminal 9 passes through analog switch 2 and flows through photodiode 16 in the forward direction, resulting in a positive voltage on the point P side of capacitor 17 and a negative voltage on the connection point side of photodiode 16 and capacitor 17. The voltage in the direction of capacitor 17
is accumulated in At time t ₁ , from Fig. 3, analog switches 1 and 8 are ON, analog switches 2 and 3 are ON, and analog switches 2 and 3 are ON.
3 is OFF, the voltage at points P and Q is GND.
Although the potential is lowered and reset, the charge in the capacitor 17 does not escape because the analog switch 33 is OFF. At time _t2 , analog switch 8
OFF, analog switches 1 and 33 turn ON, the charge in the capacitor 17 is discharged, and the voltage of the capacitor 17 is transmitted to point Q, and the DC amplifier circuit 12
The detection signal shown in FIG. 3 is outputted to the terminal 11 through the terminal 1. Next, at time _t4 , the signal detection circuit 25 outputs a detection signal to the terminal 11 in the same way as the signal detection circuit 24, and the voltage is accumulated in the capacitor 17 of the signal detection circuit 24. The amount of electricity is detected according to the shade of color in the N divided regions. In the conventional analog switch driving device, for example, the signal detection circuit 24 detects the capacitor 1 at time _t2 .
7 is discharged through the analog switch 1 and outputted from the DC amplifier circuit 12, compared to the amount of charge change depending on the amount of light charged in the capacitor 17, the analog switch 3 of the other signal detection circuits 25, 26 If the leakage current of 5 is large, is the output of the DC amplifier circuit 12 due to this leakage current?
In some cases, it may not be possible to distinguish between the output from the photoelectric conversion circuit 13 and the output from the photoelectric conversion circuit 13, resulting in poor operational accuracy and malfunction. [Summary of the Invention] This invention solves the problem that, because the leakage current of the analog switch in the conventional device is large, it is extremely difficult to accurately extract a signal proportional to the amount of light from the photoelectric conversion circuit, and the leakage current causes malfunction. This was done in order to eliminate the drawback of To provide an analog switch drive device that aims to operate accurately. [Embodiment of the Invention] An embodiment of the present invention is shown in FIG. Reference numerals 1 to 34 are the same as those in FIG. 1 described in the prior art. In FIG. 4, 35 is an analog switch for leakage current compensation that has the same configuration as analog switches 1 to 6, 36 is an analog switch whose size is 1/(N-1) of analog switch 8, and 37 is an analog switch when viewed from point P. The parasitic capacitance corresponding to the grounding of the wiring, etc. when the capacitor is connected, is equivalent to a capacitance of about 200 PF (many elements other than the one shown in FIG. 4 are connected around point P). Similarly, point R, point S
Parasitic capacitances (approximately
200PF). 40 is a capacitor, which is 1/(N-1) of the parasitic capacitance at points P, R, and S. In the circuit shown in Figure 4, for example, the time shown in Figure 3
During the period from t _1.5 to _{t 2} , analog switch 1 connected to point Q is ON, and other analog switches 3 and 5 connected to point Q are OFF, and the output from photoelectric conversion circuit 13 is Assume 10mV. Also, if the leakage current of one of the N-1 analog switches 3 and 5 in the OFF state connected to point Q is 100 nA at a positive terminal voltage of 5V, the total amount of leakage current of analog switches 3 and 5 is It becomes 100nA×(N-1). Therefore, if there are 21 analog switches connected to point Q, that is, N=21, a leakage current of 100 nA x (21-1) = 2000 nA = 2 μA flows. Since a voltage of 5V is applied to terminal 9, the resistance becomes R=5/2μA=2.5×10 ⁶ =2.5MΩ due to a current of 2μA. Therefore, if we write an equivalent circuit including the capacitance of 200PF of the parasitic capacitance 37, it will become as shown in Figure 5, and the voltage of V after switch S is turned on is determined by the time constant of CR, so

〔Effect of the invention〕

As described above, according to the present invention, a drive device using a plurality of analog switches is provided with analog switches that are always non-conductive, and the analog switches are configured to cancel each other's leakage currents, so that the analog switches operate with high precision. This has the effect of providing a switch drive device.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing a conventional analog switch drive device, Fig. 2 is an equivalent circuit diagram of the analog switch, Fig. 3 is a timing chart of the analog switch drive device shown in Fig. 1, and Fig. 4 is a diagram showing the structure of the analog switch drive device according to the present invention. FIG. 5 is a block diagram showing one implementation of such an analog switch driving device, and is an equivalent circuit diagram showing potential rise due to leakage current. In the figure, 1, 3, 5 are analog switches,
9 is a power supply terminal, 12 is a DC amplifier circuit, 35 is a non-conducting analog switch, and 40 is a capacitor.
In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Scope of Claims] 1. In a device in which a plurality of analog switches are connected in parallel to the same power source and conduction is selectively controlled, a non-conducting analog switch to which the above power supply voltage is applied is provided, and a non-conducting analog switch to which the above power supply voltage is applied is provided, An analog switch driving device characterized in that the voltage generated by the leakage current of the non-selected portion of the switch is offset by the voltage generated by the leakage current of the non-conducting analog switch. 2 The multiple analog switches and non-conducting analog switches are of the same structure, and the leakage current of the non-conducting analog switches is amplified by the number of non-selected analog switches among the multiple analog switches. An analog switch driving device according to claim 1.