JPH077916B2 - Digital-analog conversion circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention operates a duty ratio of a rectangular wave signal in correspondence with a digital amount, and performs D / A conversion in the form of an average value of the signal. A conversion circuit.

[Conventional technology]

Various methods have heretofore been adopted as a method of converting a digital amount into an analog signal. Among them, for applications such as a moving coil type analog meter that does not require a high response speed for indicating an average value, the circuit configuration is relatively simple and inexpensive, as shown in FIG. , The so-called chiyotsupa method is adopted. In the figure, 11 is a timer counter, 12 is a switching element, and 13 is a filter. In this check-up method, the fourth
As shown by the timing chart in the figure, a rectangular wave signal Vin having a predetermined amplitude Va and a duty ratio corresponding to a digital value to be converted is smoothed by a filter 13, and a desired analog signal Vout is shaped as an average value. Get at. That is, if the duty ratio during the period of taking the potential Va with respect to the period T of the rectangular wave is α, the average value Vo of the output voltage at that time is Vo.
ut is Therefore, the equation (1) represents that the average value Vout of the output voltage is proportional to the duty ratio α of the rectangular wave. Managing the duty ratio, that is, time, in the digital circuit can be easily realized by counting clocks whose frequency is stable, such as the output frequency of the crystal oscillator, so that the digital circuit can be used in accordance with the digital amount converted to an analog amount. By operating the duty ratio α of
A conversion circuit can be configured.

[Problems to be solved by the invention]

However, this method has the following problems.

That is, considering the case where a transistor is actually used as a switching element for actually generating a rectangular wave in the circuit of FIG. 3, as shown in FIG. 5 (a), the input voltage Vin is applied while the transistor is off. While it is supplied to the latter stage through the resistor R, the transistor turns on and the fourth
In the figure, the output saturation voltage of the transistor (collector-emitter voltage in this case) remains even during the period of (1-α) · T where the rectangular wave output should be a non-signal. Also, when an analog switch is used as the switching element, as shown in the equivalent circuit of FIG. 5 (b), the output voltage does not become zero when the switch is turned on because the on resistance exists, and the load resistance R and the on resistance r The voltage divided by is output. As a result, the actual D / A conversion circuit in FIG. 3 does not satisfy the equation (1).

That is, as shown in FIG. 6, if the output residual voltage is Vs and the actual output amplitude is again Va ', the average value Vout of the output voltage is And the apparent average voltage α · Va ′ becomes offset voltage Vs
It becomes a shape with the addition of. Therefore, even if the duty ratio α is set to 0, the output average voltage does not become 0. For this reason, conventionally, it has been necessary to positively apply the offset voltage to the output side so as to cancel the offset voltage Vs.

Further, in the conventional method, the amplitude Va of the rectangular wave output is a constant, and the duty ratio α is also 0 or a positive number. Therefore, as is apparent from the equation (2), the average value Vout of the resulting output voltages is obtained. The range of is such that when Vs ≤ Vout ≤ Vs + Va ': Va'> 0 (3) or when Vs + Va '≤ Vout ≤ Vs: Va'<0 (4), positive and negative values are taken as they are. It is not possible to convert the obtained digital value into an analog quantity.

Therefore, in response to such a demand, conventionally, a negative or positive offset voltage is applied to the output signal, and a constant offset amount is added to the duty ratio α corresponding to the digital amount. D / A
The conversion was realized. That is, when the offset voltage and the amount of digital offset are set as Voffset and αoffset, respectively, and the equation (2) is rewritten, the average value Vout of the output voltage is Vout = Vs + (α + αoffset) · Va′−Voffset = α · Va ′ + Vs + αoffset · Va′−Voffset ……
(5) Therefore, if αoffset and Voffset are selected so that Vs + αoffset · Va′−Voffset = 0 and α + αoffset ≧ 0, Vout can take positive and negative values in proportion to the bipolar digital amount α. Generally, the offset digital amount αoffset is a constant, and the offset voltage Voffset must be adjusted using a variable resistor or the like so as to satisfy the equation (5).

From the above, offset adjustment is indispensable in this type of D / A conversion circuit, and the application target is unipolar.
There is a problem in that the offset specification varies depending on whether the offset error is compensated in the D / A conversion or the zero potential is adjusted in the bipolar D / A conversion.
Since this problem is a problem derived from each D / A conversion circuit, when a plurality of D / A conversion circuits are provided, the offset adjustment element described above is provided for each circuit and the adjustment is performed. It had a problem that it had to be done.

Therefore, the present invention eliminates the offset error without providing an offset adjustment element in the circuit and can convert the bipolar digital amount into an equivalent analog amount.
It is intended to provide a D / A conversion circuit.

[Means for solving problems]

In order to achieve the above object, the present invention operates a duty ratio of a rectangular wave signal in correspondence with a digital amount, obtains an analog amount in the form of an average value of the signal, and performs a digital / analog conversion circuit. A first signal generating means for outputting a first rectangular wave signal having a predetermined cycle, an amplitude Va ′, and a duty ratio α ₁ ;
Second rectangular wave signal having the same period and amplitude Va ′ as that of the rectangular wave signal of No. 2, and the duty ratio α ₀ of the second rectangular wave signal can be varied; The signal subtracting means for calculating the difference between the average values of the output signals from the respective signal generating means and the duty ratio α ₀ of the output signal of the second signal generating means are calculated, and the calculated duty ratio is digitally calculated. Arithmetic control means set in the second signal generating means as corresponding to the digital amount to be analog-converted, and the average value E ₁ of the output signals of the first signal generating means and the second signal. When the relationship of E ₁ −E ₀ = (α ₁ −α ₀ ) · Va ′ holds between the average value E ₀ of the output signal of the generating means, the arithmetic control means calculates by the signal subtracting means. The resulting difference value is digital / analog converted. The duty ratio α ₀ of the output signal of the second signal generating means is calculated so as to obtain a desired analog quantity obtained as a result of the conversion, and the calculated duty ratio corresponds to the digital quantity to be digital-to-analog converted. The setting is made in the second signal generating means.

[Action]

The (2) and the average output voltage E ₀ for duty ratio alpha ₀ in the circuit of formula, the difference E ₁ -E ₀ of the average output voltage E ₁ for duty ratio alpha ₁ by the same circuit using an operational amplifier The average output voltage of the configured subtraction circuit becomes E ₁ −E ₀ = (Vs + α ₁ , Va ′) − (Vs + α ₀ · Va ′) = (α ₁ −α ₀ ) · Va ′ (6), and the output It does not depend on the residual voltage Vs. Moreover, if the duty ratios α ₀ and α ₁ are appropriately selected, α ₁ −
Since α ₀ can take a positive or negative value, a positive or negative output voltage without offset error can be obtained without providing the offset voltage Voffset as shown in the equation (5).

In view of this point, the present invention determines the difference in average output voltage between a rectangular wave signal operating at a predetermined fixed cycle and duty ratio and a rectangular wave signal operating at the same cycle and amplitude. By utilizing the fact that it is proportional to the deviation of the duty ratio and operating the error of the duty ratio according to the digital amount to be converted and converting the digital amount into a voltage (analog amount), there is no offset error of both polarities. D /
It is intended to provide an A conversion circuit.

〔Example〕

FIG. 1 is a block diagram showing an embodiment in which the present invention is applied to two digital-analog conversion circuits, and FIG. 2 is a waveform diagram showing its operation waveform.

In FIG. 1, a counter 1A counts a predetermined count value N ₀ with a reference clock f having a stable frequency (FHz) with high accuracy and alternately turns on and off its output signal S ₀ . The counter 1B and the counter 1C operate in synchronization with the rising edge of the output signal S ₀ of the counter 1A to count the count values N ₁ and N ₂ set via the bus 3 from the arithmetic processing unit CPU 2 such as a microcomputer. Start
The outputs S ₁ and S ₂ are output from the count start to the end. The outputs S _{0 to} S ₂ of the counters 1A to 1C are applied to the analog switches 4A to 4C, respectively. Analog switch 4A-4C
Applied to the subsequent circuit _- while signals S ₀ to S ₂ is 'High' level, a negative reference voltage -V _R ef. Therefore, the analog switch 4A supplies the positive and negative reference voltages + V _R ef ₊ and −V _R ef ₋ while the signal S ₀ is at the “High” level, that is, the period required for the counter 1A to count the count value N _0. Only the positive reference voltage + V _R ef ₊ is applied to the circuit in the subsequent stage while being applied to the circuit in the subsequent stage, and while the signal S ₀ is at the “Low” level, that is, the period required for the counter 1A to count the count value N ₀ . Applied, and its inverted output is output by the operational amplifier 5A.
And 5C. Similarly, analog switches 4B and 4C
Means that while the counter output signals S ₁ and S ₂ are at the'High 'level, that is, the counters 1B and 1C are counting values respectively.
During application of a subsequent stage of the circuit, subsequently signals S ₁ and S ₂ are 'Low' level _- between the positive and negative reference voltages + V _R ef ₊ and -V _R ef required to count the N ₁ and N ₂ , Ie counter
1A is count (2N ₀ -N ₁₎ or (2N ₀ -N ₂₎ period required to count, to apply a positive reference voltage + V _R ef ₊ only to a subsequent circuit. The operational amplifiers 5B and 5C correspond to the outputs of the analog switches 4B and 4C and the output of the operational amplifier 5A, respectively, and the positive reference voltage V _R ef ₊ applied via a resistor, and the arithmetic sum smoothed by the filter element. Output inverted output.

Assuming that the time required to count the count values N ₀ , N ₁ and N ₂ is t ₀ , t ₁ , t ₂ , respectively, t ₀ = N ₀ / F (7) t ₁ = N ₁ / F (8) t ₂ = N ₂ / F (9). Therefore, in FIG. 1, if the average output voltage V ₀ of the operational amplifier 5A is the on resistance of the analog switch 4A, r ₀ , Is required as. At the same time, the average output voltages V ₁ and V ₂ of the operational amplifiers 5B and 5C are Becomes Here, since it is easy to select the same resistance value by using high-precision resistors, R _0P ＝ R _1P ＝ R _2P …… (13) R _0N ＝ R _1N ＝ R _2N …… (14 ) R _0F = R ₁₀ = R _1F = R ₂₀ = R _2F (15) Also, for the on resistances r ₀ , r ₁ , r ₂ of the analog switch, it is practically possible to select the same value by using the elements having the same characteristics.
In particular, as an analog switch commercially available for such an application, in an integrated circuit in which a plurality of analog switch elements are enclosed in one package, since the integrated circuits are formed on the same semiconductor wafer, The deviation of the ON resistance value between the integrated analog switches is usually extremely small as compared with the error of the absolute value. Accordance connexion, it is possible to _{r 0 ≒ r 1 ≒ r 2} ...... (16), where _{R P = R 0P = R 1P} = R 2P ...... (17) R N = R 0N = R 1N = R 2N …… (18) R _F ＝ R _0F ＝ R ₁₀ ＝ R _1F ＝ R ₂₀ ＝ R _2F …… (19) R ＝ r ₀ ＝ r ₁ ＝ r ₂ …… (20) and formula (11), ( 12) is organized, Is obtained. Therefore, V ₁ and V ₂ are t ₁ −t ₀ and t ₂ −t, respectively.
_It is proportional to ₀ . Therefore, the count values N ₁ and N ₂ of the counters 1B and 1C should be converted into the analog quantity D.
_{If 1} and D ₂ are selected as shown in the following equations (23) and (24), N ₁ = N ₀ + κ · D ₁ …… (23) N ₂ ＝ N ₀ + κ · D ₂ …… (24 ). Here, t ₁ −t ₀ and t ₂ −t ₀ can be rewritten as follows: t ₁ −t ₀ = κ ・ D ₁ / F …… (25) t ₂ −t ₀ = κ・ D ₂ / F ・ ・ ・ (26), therefore the average output voltage of the operational amplifiers 5B and 5C
V ₁ and V ₂ are proportional to the digital amounts D ₁ and D ₂ .

〔The invention's effect〕

According to the present invention, in D / A conversion by the chip method, a circuit for outputting a reference rectangular wave signal is provided, and a difference between the reference rectangular wave signal and a rectangular wave signal generated by a circuit having a circuit constant equivalent to that of the reference rectangular wave signal is provided. By taking the above, it is possible to cancel the offset error of the analog conversion amount due to the output saturation voltage of the switching element, which is an indispensable constituent element in this kind of circuit, and the ON resistance component without providing an adjusting element on the circuit. In addition, it is possible to realize a bipolar D / A conversion operation by making it correspond to the difference in the duty ratio of two rectangular wave signals even for the bipolar digital amount.

Moreover, whether the D / A conversion circuit according to the present invention is operated as a unipolar conversion circuit or a bipolar conversion circuit is determined only by how to obtain the duty ratio of two rectangular wave signals. A highly versatile D / A conversion circuit that does not depend on the type of conversion amount can be configured.

Further, even when a plurality of D / A conversion circuits are provided, taking the circuit of FIG. 1 as an example, each D / A conversion circuit to be added has one counter, one analog switch, and one subtraction circuit using an operational amplifier. The above functions can be realized simply by adding them.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a waveform chart of each part for explaining the operation, and FIG. 3 is a tipper type.
FIG. 4 is a schematic view showing a conventional example of a D / A conversion circuit, FIG. 4 is a waveform chart of each part for explaining the operation, FIG. 5 is a circuit diagram showing a concrete example of the switching element shown in FIG. 3, and FIG. FIG. 4 is an operation waveform diagram showing a case where the output residual voltage of the switching element is taken into consideration in FIG. 3. Description of symbols 1A, 1B, 1C ... Counter, 2 ... Arithmetic processing unit (CPU),
3 ... Bus, 4A, 4B, 4C ... Analog switch, 5A, 5B, 5C
...... Operational amplifier, 11 …… Timer counter, 12 …… Switching element, 13 …… Filter.

Claims (1)

[Claims] 1. A digital device for operating a duty ratio of a rectangular wave signal corresponding to a digital amount to obtain an analog amount in the form of an average value of the signal and performing digital / analog conversion.
An analog conversion circuit, comprising: first signal generating means for outputting a first rectangular wave signal having a predetermined cycle, an amplitude Va ′, and a duty ratio α ₁ ;
Second rectangular wave signal having the same period and amplitude Va ′ as that of the rectangular wave signal of No. 2, and the duty ratio α ₀ of the second rectangular wave signal can be varied; The signal subtracting means for calculating the difference between the average values of the output signals from the respective signal generating means and the duty ratio α ₀ of the output signal of the second signal generating means are calculated, and the calculated duty ratio is digitally calculated. Arithmetic control means set in the second signal generating means as corresponding to the digital amount to be analog-converted, and the average value E ₁ of the output signals of the first signal generating means and the second signal. When the relationship of E ₁ −E ₀ = (α ₁ −α ₀ ) · Va ′ holds between the average value E ₀ of the output signal of the generating means, the arithmetic control means calculates by the signal subtracting means. The resulting difference value is digital / analog converted. The duty ratio α ₀ of the output signal of the second signal generating means is calculated so as to obtain a desired analog quantity obtained as a result of the conversion, and the calculated duty ratio corresponds to the digital quantity to be digital-to-analog converted. The digital-analog conversion circuit is characterized by being set in the second signal generating means.