JPH05191279A - A/d converter - Google Patents

Abstract

PURPOSE:To control freely a voltage applied to an analog input terminal not in use by employing a power supply for a selector switch element independently of a drive power supply. CONSTITUTION:When selection signals X0 and X1 whose levels are respectively H, L are given to an A/D converter 10 via inverters 15a, 15b, a transmission gate 14a is closed and a transmission gate 14b is opened. In this case, even when a voltage of 0-AVCC is inputted to an analog input terminal AN1, there is no problem, but when a voltage in excess of the drive power supply voltage AVcc is inputted to the analog input terminal AN1, it becomes a cause to malfunction. Then the voltage of the power supply to activate the transmission gates 14a, 14b at the analog input terminals AN0, AN1 is set to voltage terminals AVcc0, AVcc1. When the voltages AVcc, AVcc0 are set to 5(V) and the voltage AVcc1 is set to 6(V), a voltage input up to 6(V) is attained for the analog input terminal AN1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A / D converter for converting an analog voltage value into a digital value.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a conventional A / D converter. In FIG. 2, 10 is an A / D converter,
Reference numeral 11 is a comparator, 12 is a conversion register in which an A / D conversion result is entered, 13 is a ladder resistor, 14 is a transmission gate for selecting an analog input, X ₀ and X ₁ are selection signals for turning on / off the switch of the transmission gate 14, AN ₀ and AN ₁ are analog input terminals for inputting an analog voltage, and Vref is a reference voltage. It should be noted that FIG. 2 shows a two-channel A / D conversion device for ease of explanation. 3 is a sectional view of the transmission gate 14 of FIG. 2, where AN _n is an input terminal, X _n is a selection signal, P (+) is a P-type region, and N (+) and N (−).
Is an N-type region.

Next, the operation of the conventional A / D converter will be described. At present, most of the A / D conversion devices formed in semiconductor devices use the successive approximation method. That is, for example, the selection signals X ₀ = 'H', X ₁ =
When the analog input terminal AN ₀ is selected by “L” (at this time, the analog input terminal AN ₁ is not selected), the analog voltage is input to the comparator 11 via the analog input terminal AN ₀ . The input analog voltage is compared with the reference voltage of the ladder resistor 13 by the comparator 11. As a result, the input analog voltage is converted by the conversion register 12 into a digital value with a resolution of n bits between 0 and Vref volts. For example, if the conversion register 12 has an 8-bit configuration (n = 8),
It can be converted into a digital value from "00" to "FF". A detailed description of the conversion will be omitted because it is not directly related to the present invention. Further, when the A / D converter is built in a microcomputer or the like, the power source of the microcomputer itself and the power source of the A / D converter are separated from each other for accuracy. For example, A / in FIG.
When the D converter is built into the microcomputer,
In addition to the drive power source AV _cc for the A / D converter, a power source for the microcomputer was separately provided.

[0004]

Since the conventional A / D converter is configured as described above, when the analog input terminal AN ₀ of FIG. 2 is selected and the analog input terminal AN ₁ is not used. However, there is a problem that a voltage exceeding the drive power source voltage AV _cc cannot be applied to the analog input terminal AN ₁ . That is, since the analog input terminal AN ₁ is connected to the transmission gate 14 which is a switching element, the island potential (voltage applied to the PMOS substrate)
Is AV _cc , and if a voltage exceeding this is applied to the analog input terminal AN ₁ , a large current flows in the forward direction, which causes latch-up and the like, which causes malfunction.

The present invention has been made in order to solve the above problems, and provides an A / D converter which does not cause a problem even when a voltage higher than the voltage of the driving power supply is applied to an unused analog input terminal. The purpose is to

[0006]

The A / D converter according to the present invention, as shown in FIG. 1, includes selection switch elements (transmission gates 14a, 14b) to which analog voltages are respectively supplied and reference value output means. (Ladder resistance 1
3) A comparator 11 for comparing the reference voltage from the above and an analog voltage input via any of the selection switch elements, a conversion register 12 for displaying the output of this comparator as a digital value, and a drive power source AVcc. In the A / D conversion device including the A / D converter 10 having the above, a power supply independent of the drive power supply is set as the power supply of the selection switch element. In this way, a power supply voltage terminal for setting an independent power supply is provided for each analog input terminal corresponding to the selection switch element.

[0007]

In the A / D converter according to the present invention, a power source independent of the drive power source is set as the power source of the selection switch element, and a power source is provided for each analog input terminal. The power supply voltage can be changed for each terminal, and a voltage higher than the voltage of the drive power supply can be applied.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an A / D converter according to an embodiment of the present invention, which is a device capable of 2-channel analog input. In FIG. 1, 10 is an A / D converter, 11 is a comparator, 12 is a conversion register, 13 is a ladder resistor as a reference value output means, and 14a and 14b are selection switches for selecting an analog input terminal to which an analog voltage is input. Transmission gate as an element,
AV _cc0 and AV _cc1 are transmission gate PMO
A power supply voltage terminal for the S substrate (island), AV _cc is the voltage of the drive power supply of the A / D converter, and AN ₀ and AN ₁ are analog input terminals. Power supply voltage terminals AV _cc0 , AV _cc1
Are independent of the drive power supply voltage AV _cc, and an arbitrary voltage is set to each of the power supply voltage terminals.

Next, the operation of this embodiment will be described. Since the A / D conversion method itself by the A / D conversion device 10 is the same as the conventional method, description thereof will be omitted. The selection signals X ₀ = 'H', X ₁ = are sent to the A / D converter 10.
When "L" is applied via the inverters 15a and 15b, the transmission gate 14a is turned on and the transmission gate 14b is turned off. At this time, even if a voltage of 0 to AV _cc is input to the analog input terminal AN ₁ ,
Although there is no problem, if a voltage of the power supply exceeding the drive power supply AV _cc is input to the analog input terminal AN ₁ , it will cause a malfunction for the reasons described above. Therefore, the respective power supplies for operating the transmission gates 14a, 14b of the analog input terminals AN ₀ , AN ₁ are set to the power supplies (voltage terminals) AV _cc0 , AV _cc1 . (For example, the power supply voltage terminal AV _{cc0 is set} to V1 (v) and the power supply voltage terminal is set to V2 (v).
For example, when the analog input terminal AN ₀ is used and the analog input terminal AN ₁ is unused, AV _cc = AV _cc0
= 5 (v) and AV _cc1 = 6 (v), a voltage of up to 6 (v) can be input to the analog input terminal AN ₁ .

In the above embodiment, the case of an A / D converter capable of analog input of 2 channels has been described, but the number of channels is not limited to that in the above embodiment. If the number of channels increases or decreases,
Power supply voltage terminals may be provided in accordance with the number of channels. Therefore, in general, n power supply voltage terminals should be provided for each of the n-channel analog inputs.

[0011]

As described above, according to the present invention, since the power source of the selection switch element is independent of the drive power source, the voltage applied to the unused analog input terminal can be freely controlled. effective.

[Brief description of drawings]

FIG. 1 is a block diagram of an A / D conversion device showing an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional A / D conversion device.

3 is a cross-sectional view of a transmission gate in the device of FIG.

[Explanation of symbols]

 10 A / D converter 11 Comparator 12 Conversion register 13 Ladder resistance 14a, 14b Transmission gate

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[Procedure amendment]

[Submission date] September 4, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction content]

[0004]

Since the conventional A / D converter is configured as described above, when the analog input terminal AN ₀ of FIG. 2 is selected and the analog input terminal AN ₁ is not used. However, there is a problem that a voltage exceeding the drive power source voltage AV _cc cannot be applied to the analog input terminal AN ₁ . That is, the analog input terminal AN _1, since it is connected to the transmission gate 14 is a switch element, the island potential (voltage applied to the PMOS board)
Is AV _cc , and if a voltage exceeding this is applied to the analog input terminal AN ₁ , a large current flows in the forward direction, which causes latch-up and the like, which causes malfunction.

Claims (1)

[Claims] 1. A selection switch element to which an analog voltage is respectively supplied, a comparator for comparing a reference voltage from a reference value output means, and an analog voltage input via any one of the selection switch elements, In an A / D converter comprising an A / D converter having a conversion register for displaying the output of a comparator as a digital value and a drive voltage, a power supply independent of the drive power supply is set as a power supply for the selection switch element. A / characterized by having done
D converter.