JPH0685676A - A/d conversion circuit of r-string system - Google Patents

Abstract

PURPOSE:To accelerate conversion speed by connecting a gate consisting of only H-channel transistors to the parting point of a resistor string, and controlling the gate by outputting a value of more than source voltage from a tap decoder. CONSTITUTION:A transfer gate is comprised of only the N-channel transistors M1-Mn. and the gates M1-Mn are selected by the tap decoder 5. The output of the decoder 5 is boosted to the value more than a source voltage by a booster circuit 6 to reduce the on-resistance of the gate. Potential selected by the decoder 5 is compared with an analog voltage inputted from an external terminal by a comparator 4, and a result is written on a register 2. Such comparison is continued by rewriting the value of the decoder 5 until a conversion result completion signal is sent out from a control circuit 1. Such a series of execution time is dependent on the on-resistance of the transistor and load capacity to the comparator 4. Thereby, fast conversion can be performed, and also, circuit configuration can be simplified.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an R / string type A / D conversion circuit, and more particularly to an A / D conversion circuit provided with a selecting means for selecting a division potential of an R string.

[0002]

2. Description of the Related Art A conventional R-string type A / D conversion circuit is generally an input system sample-hold circuit or comparator, an output system register or control circuit, and a resistor string or tap for supplying a reference voltage to the comparator. And a decoder.

FIG. 4 is an A / D conversion circuit diagram of the R string system showing an example of such a conventional technique. As shown in FIG.
The conventional R string A / D conversion circuit includes a sample hold circuit 3 that samples and holds an analog signal input through an external terminal, a comparator 4 that compares the output of the sample hold circuit 3 with a reference voltage, and a comparator 4. 2 for storing the output of the control circuit, a control circuit 1 for controlling the register 2 according to an instruction from the bus, resistor strings R _{1 to} R _n for creating a reference voltage, a tap decoder 5 for selecting the reference voltage, and various types. Of the transistors N _{1 to} N _m . The analog signal input via the external terminal is sampled by the sample hold circuit 3 and supplied to the comparator 4. On the other hand, the reference voltage is obtained by dividing a resistor string connected in series between the AVref terminal and GND. The potential is selected by register 2
This is performed by turning on / off a transfer gate composed of an N-channel transistor, a P-channel transistor, and a non-doped transistor by a tap decoder 5 controlled by. At this time, the ON resistance of the transfer gate varies depending on the type of the transfer gate.

FIG. 5 is a characteristic diagram of the input voltage and the ON resistance of the various transistors shown in FIG. As shown in FIG. 5, the ON resistance characteristic of the N-channel transistor is similar to that of the non-doped transistor, but the P-channel transistor has the opposite (reverse) characteristic as compared with the N-channel transistor.

First, in order to increase the speed at the time of selection, it is necessary to lower the ON resistance of the transfer gate, and it is necessary to select an element according to the potential. Therefore, the P-channel transistor is used at a high potential, the N-channel transistor is used at a low potential, and the undoped transistor is used at an intermediate potential.

Next, the potential selected by the tap decoder 5 is compared with a voltage obtained by holding the analog voltage input from the external terminal by the sample hold circuit 3 by the comparator 4, and the result is written in the register 2.

Further, the value of the tap decoder 5 is rewritten and the comparison is continued until the control circuit 1 outputs a signal indicating the end of the conversion result. When the conversion end signal is output from the control circuit 1, the register 2 can obtain a digital value that has been A / D converted via the bus.

As described above, in the conventional R-string type A / D conversion circuit, the P-channel transistor, the N-channel transistor and the non-doped transistor are selectively used according to the potential, so that the A / D conversion is realized at a relatively high speed. It is possible.

[0009]

The conventional R string A / D conversion circuit described above is constructed by combining a P-channel transistor, an N-channel transistor and a non-doped transistor according to the potential.
Therefore, there is a drawback that the circuit configuration and the mask pattern are complicated. In addition, even in a configuration using an element according to the potential to increase the speed at the time of selection,
There is a drawback in that the performance of the device is limited and it is difficult to realize faster switching.

An object of the present invention is to provide an A / D conversion circuit of the R string system which can simplify the circuit structure and the mask pattern and can realize switching at higher speed.

[0011]

An R / string type A / D conversion circuit according to the present invention comprises a sample hold circuit for inputting an analog signal to sample and hold it, and a plurality of resistance elements for dividing a reference voltage in series. Connected resistor strings, a plurality of transfer gates consisting of only N-channel transistors, one end of which is commonly connected and the other end of which is connected to a dividing point of the resistor string, and a tap for controlling the plurality of transfer gates A decoder and a booster circuit connected to the tap decoder,
A comparator that compares the output of the sample hold circuit and a reference voltage obtained from one end of the plurality of transfer gates that are commonly connected; and a register that stores the output of the comparator circuit and digitally outputs it, and controls the tap decoder. The booster circuit is configured to select the N-channel transistor that constitutes the transfer gate according to the boosted signal.

[0012]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is an R-string type A / D conversion circuit diagram showing an embodiment of the present invention. As shown in Figure 1,
The A / D conversion circuit according to the R string method of this embodiment is
A control circuit 1 for performing conversion control by sending a conversion start signal and a conversion end signal or the like by a signal from the bus, a register 2 for sending the A / D conversion result to the bus by the conversion end signal from the control circuit 1, and an external device. A sample and hold circuit 3 that inputs and holds an analog signal from a terminal, a comparator 4 that compares the output of the sample and hold circuit 3 and a selected reference voltage, and sends the comparison result to a register 2, an AVref terminal and ground. transfer gates M ₁ ~M connected a resistor string comprising resistors R ₁ to R _n which are connected in series, and commonly connected at one end to the other input of the comparator 4 and the other end to the dividing point of each resistor string between _n, a tap decoder 5 for selecting the transfer gate M ₁ ~M _n, the Tappude And a booster circuit 6 for the output of the over da 5 than the supply voltage. In particular, Torajisuta M ₁ ~M _n forming the transfer gate is used only N-channel transistors.

In this A / D conversion circuit, the output from the tap decoder 5 is boosted by the booster circuit 6 to the power supply voltage or more, so that the selected transfer gate (M ₁ to
All M _n ) can be composed of N-channel transistors.

FIG. 2 is a characteristic diagram of the gate voltage and current of the N-channel transistor shown in FIG. As shown in FIG. 2, this voltage-current characteristic, that is, the relationship between the gate voltage V _G of the N-channel transistor and the current I _{D is} as follows.
_The higher _G is, the lower the ON resistance of the transistor and the higher the current I _D.

FIG. 3 is a characteristic diagram of the input voltage and ON resistance of the N-channel transistor shown in FIG. As shown in FIG. 3, even if all the transistors of the transfer gate to be selected have N-channel transistors, the gate voltage V _G of the ON resistance characteristic is V _G = b by the booster circuit 6.
Alternatively, as in the case of c, since the voltage is boosted to the power source voltage or higher, whether the input voltage is high or low,
ON resistance is small.

When a high input voltage is selected by the N-channel transistor, the voltage drops by the threshold value (V _TH ) of the gate voltage, but there is no problem if the boosted voltage is set to the power supply voltage + V _TH or more. .

In short, the potential selected by the tap decoder 5 is compared with the analog voltage input from the external terminal by the comparator 4, and the result is written in the register 2. The value of the tap decoder 5 is rewritten until the conversion result end signal is transmitted from the control circuit 1, and the comparison is continued. This series of execution times depends on the ON resistance of the N-channel transistor forming the trough gate and the load capacity up to the comparator 4, and the conversion speed is also determined by these. Therefore, in this embodiment, the gate O
Since the N resistance becomes smaller, the conversion speed also becomes faster. Moreover, since only N-channel transistors are used, circuit design becomes easy and the area can be reduced.

[0018]

As described above, since the A / D conversion circuit of the R string system of the present invention has the gate means and the booster circuit which are formed only by the N-channel transistor, the power supply voltage from the tap decoder is higher than the power supply voltage. Since the value can be output, the ON resistance of the transistor can be reduced, and the conversion speed can be increased. Further, when the present invention is realized in a mask pattern, the non-doping process can be eliminated by constructing the N-channel transistor only, and since there is no P-channel transistor, it is not necessary to provide an element interval and the area can be reduced. There is an effect that it can be designed small and the circuit configuration and mask pattern can be simplified.

[Brief description of drawings]

FIG. 1 is an R-string type A showing an embodiment of the present invention.
It is a / D conversion circuit diagram.

FIG. 2 is a characteristic diagram of gate voltage and current of the N-channel transistor shown in FIG.

3 is a characteristic diagram of an input voltage and an ON resistance of the N-channel transistor shown in FIG.

FIG. 4 is an R-string type A / D conversion circuit diagram showing a conventional example.

5 is a characteristic diagram of an input voltage and an ON resistance of the transistor shown in FIG.

[Explanation of symbols]

1 Control Circuit 2 Register 3 Sample and Hold Circuit 4 Comparator 5 Tap Decoder 6 Booster Circuit M _{1 to} M _n N Channel Transistor R _{1 to} R _n Resistance

Claims (1)

[Claims] 1. A sample hold circuit for inputting an analog signal to sample and hold, and a resistor string in which a plurality of resistor elements for dividing a reference voltage are connected in series,
A plurality of transfer gates, each of which has one end connected in common and the other end connected to a division point of the resistor string, and each of which includes only an N-channel transistor, a tap decoder for controlling the plurality of transfer gates, and the tap decoder. And a comparator for comparing the output of the sample and hold circuit and a reference voltage obtained from one of the commonly connected ends of the plurality of transfer gates, and the output of the comparator circuit for storing and digitally outputting A register for controlling the tap decoder, and the transfer gate is configured by a signal boosted from the booster circuit.
An A / D conversion circuit according to the R string method, characterized in that a channel transistor is selected.