JPH0131967Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a multi-point current measuring device that converts multi-channel signal currents into voltages and measures the voltages by selectively switching the signals, thereby determining the signal current value of each channel. It is.

For example, X-ray CT (Computerized Tomography)
The apparatus is configured to irradiate an object to be measured with X-rays, detect the transmitted X-rays with a number of ionization chambers, and obtain projection data regarding the object based on the signals. In this case, the device uses a multi-point current measuring device that measures the output current of each multi-channel ionization chamber. When measuring a large number of ionizing currents corresponding to the intensity of transmitted X-rays obtained from an X-ray ionization chamber in this way, each ionizing current (hereinafter generally referred to as signal current) is charged to a capacitor, and the charged voltage is calculated. A possible method is to measure while making selections using a changeover switch. However, in a method in which the charge voltage of the capacitor is simply read by a changeover switch, crosstalk occurs due to the distributed capacitance of the circuit, which is undesirable.

The present invention attempts to provide a means for eliminating the adverse effects of crosstalk caused by the above-mentioned distributed capacitance.

FIG. 1 is a configuration diagram of main parts showing an embodiment of a multi-point current measuring device according to the present invention. In the same figure, I ₁
~I _o is the signal current to be measured. C ₁ -C _o are capacitors that convert each signal current I ₁ -I _o into voltage. S ₁ to S _o represent changeover switches and each capacitor
It selects the voltages from C ₁ to _Co and transmits them to the next stage. Note that the changeover switches S ₁ to _{S o} may be replaced with multiplexers instead of a plurality of switch elements as shown in the figure. Cx is an equivalent representation of distributed capacitance occurring in the input circuit of amplifier U, which will be described later. Sr represents a reset switch and is a switch provided according to the present invention. U represents an amplifier, and the amplification degree is determined by resistors _R1 and _R2 .

Capacitors C ₁ to _{C o} are connected to each input channel terminal 1
~n and circuit ground. Each input channel terminal 1 to n is connected to the amplifier U via each changeover switch S ₁ to _{S o} and to circuit ground via a contact of a reset switch Sr.

The operation of the apparatus shown in FIG. 1 configured as above will be explained with reference to FIG. 2. FIG. 2 is a diagram showing the timing of the changeover switches S ₁ -S _o and the reset switch Sr. On the left side of each waveform, the name of the switch element for the operation corresponding to that waveform is shown.
It is assumed that the switch is on during the "high" level period of the waveform in FIG. 2, and is off during the "low" level period.

First, the operation when there is no reset switch Sr will be explained. It is assumed that in the initial state, the changeover switches S ₁ to S _o are all off, and the distributed capacitance Cx is not charged with any electric charge.

(a) A signal current is applied, and charge voltages of E ₁ and E _o are generated in each of the capacitors C ₁ to C _o , respectively.

(b) Next, only the changeover switch _S1 is turned on, and the voltage _e1 shown in equation (1) is introduced into the amplifier U.

e ₁ =C ₁ /C ₁ +Cx·E ₁ (1) Since the voltages of the capacitors C ₁ to C _o are introduced into the non-inverting input terminal of the amplifier U, they are not affected by the input impedance.

(c) The voltage e ₁ in equation (1) is amplified to an appropriate size by the amplifier U, and the charge voltage E ₁ of the capacitor C ₁ is
measurements are taken. That is, the signal current _I1 input to the first channel is measured.

(d) Next, the changeover switch S ₁ is turned off, but the voltage e ₁ of the above equation (1) remains in the distributed capacitance Cx.

(e) Next, when only the changeover switch S ₂ is turned on, the voltage e ₂ shown in equation (2) is introduced into the input of the amplifier U.

e ₂ = C ₂ /C ₂ +Cx・E ₂ +1/C ₂ +Cx ・C ₁・Cx/C ₁ +CxE ₁ (2) The value measured at this time includes the capacitor C ₁ as shown in equation (2). It includes a charge voltage _E1 and becomes crosstalk. Assuming that _the capacitances _of capacitors _{C 1 to Co} This will be included in the measurement as crosstalk. Similarly, to measure the charge voltage of capacitors C ₃ to C _o ,
All include errors due to crosstalk.

As mentioned above, the reset switch according to the present invention
Without Sr, crosstalk occurs due to the distributed capacitance Cx, which is undesirable.

Next, the operation when a reset switch Sr is provided will be explained. Regarding (a) already mentioned, the operation is exactly the same. Next, as shown in FIG. 2, only the changeover switch _S1 is turned on, and the signal current _I1 input to the first channel is measured by the same operation as in (b) and (c) described above.

The operations up to this point are the same as those described in (a) to (c) above, but the operations from the point (d) onward are different in the present invention. That is, in the present invention, as shown in FIG. 2, the reset switch Sr is driven to be turned on during the period between when the changeover switch _S1 is turned off and when the next changeover switch _S2 is turned on. Therefore, the voltage _e1 shown in equation (1) that has been charged to the distributed capacitance Cx is discharged by the reset switch Sr.

Since the changeover switch S ₂ is turned on in a state where the distributed capacitance Cx is not charged with electric charge, the voltage e ₂ ' shown in equation (3) is introduced into the input of the amplifier U.

e ₂ ′=C ₂ /C ₂ +Cx・E ₂ (3) In other words, the effect of voltage E ₁ that occurred in equation (2) is removed in this invention, and the signal current of the object to be measured
A voltage E ₂ based only on I ₂ is measured.

As described above, in the present invention, during the period between measuring the voltage of a certain capacitor C _K and measuring the voltage of the next capacitor C _K+1 , the reset switch Sr is turned on and the charge of the distributed capacitor Cx is Since it is discharged, crosstalk does not occur.

In addition, in Figure 2, the reset switch is turned off during the entire period until the changeover switch changes to the next switch.
Although Sr is shown to be on, the present invention is not limited to this. That is, it is sufficient to turn on the reset switch Sr only during a period during which the charges accumulated in the distributed capacitance Cx can be discharged.

In addition, in Fig. 2, the reset switch Sr
is connected to the non-inverting input terminal of the amplifier U, but the present invention will work even if it is connected to the inverting input terminal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the main parts of an embodiment of a multi-point current measuring device according to the present invention, and FIG. 2 is a diagram showing the timing of each switch. I ₁ ~ _{I o} ... Signal current, C _{1 ~ Co} ... Capacitor,
S ₁ to S _o ...changeover switch, Sr...reset switch, U...amplifier, Cx...distributed capacitance.

Claims (1)

[Claims for Utility Model Registration] A multi-point current measuring device that converts signal currents of multiple channels into voltages and measures the voltages by selectively switching the signals, thereby determining the signal current value of each channel. A plurality of capacitors are provided for each capacitor to convert signal current into voltage, a changeover switch that can selectively select the voltage of the capacitor, and a voltage of the capacitor introduced through the changeover switch. an amplifier for amplification, and a reset switch connected between the input stage of the amplifier and circuit ground, and the reset switch operates during the period between introducing the voltage of one capacitor and the next capacitor to the amplifier by the changeover switch. A multi-point current measurement device that turns on.