DE3814941C1 - Method for analog/digital conversion - Google Patents

Abstract

The invention relates to a method for generating a digital signal, the value of which is in each case proportional to two mutually independent analog control signals. One analog control signal + -uE can contain the information in the form of an amplitude modulation and the other control signal can contain information in the form of a frequency modulation in the form of a continuous pulse with the period To. The method should be implementable with little expenditure. It is proposed that a control circuit controls a discharge phase T2' in accordance with the equation (+ -iE + iR) . To = iC . T2', in such a manner that the electrical discharge iC . T2' proportional to T2' corresponds to the charge quantity (+ -iE + iR) . To.

Description

The invention relates to a method for converting a analog input signal into a digital output signal with intermediate conversion into a plus pause difference modulated (PPDM) signal.

Among other things, a number of integrating conversion methods are known which convert an analog signal into a digital signal. In the specialist book "Data Converter" by Horst Zander from the Frankfurt specialist publishing house, the following are described:
The dual-slope method, which integrates the input signal over a constant time and then triggers a down-integration. The main disadvantages of the method are the short integration time relative to the total conversion time and the dependence of the conversion time on the input signal.

The charge balancing process, in which each pulse of a continuous high-frequency clock pulses by means of a charge compensation circuit a sign is assigned. Since the high-frequency clock pulses are used as the output signal Possible uses of the method are limited.

The voltage / frequency procedure is integrated during a constant time the sum signal from the input voltage and a reference voltage and then integrated with the input signal. As major drawbacks The principle of the high-frequency output pulse applies to the method as well as the implementation time, which varies within wide limits.

A method for integrating A / D conversion is to be created that avoids the disadvantages of the known methods.

This object is achieved by the features of patent claim 1 solved. Useful sub-claims can be found in the subclaims.  

The PULSE PAUSE DIFFERENCE MODULATION (hereinafter PPDM) is a Method for coupling analog signal sources to digital acquisition units and a universal implementation process that on the equivalent link of two different ones Waveforms based that control function independently exercise. The most frequently used control signals in sensors occurring waveforms, namely

• 1. the analog voltage signal, the measured values or information in the form of an amplitude modulation,
• 2. the analog or quasi-digital pulse signal, the measured values or Information in the form of period or frequency modulation contains.

PPDM generates an output pulse that has the following properties having:

• 1. The sum of the pulse duration (high phase) and pause duration (low phase) is equal to the period of the input pulse.
• 2. The difference between the pulse and pause duration is
  • a) proportional to the analog input voltage
  • b) proportional to the period of the input pulse.

This relationship can be used in conjunction with an up / down counter to a very simple and highly accurate analog / digital conversion use.

In addition to the pulse-pause difference-modulated output signal, that controls the counting direction, just a clock pulse constant and higher frequency, of which the one to be modulated Input pulse is derived.

A large number of specific applications can be represented. In the main function "analog / digital coupling" sends the digital Unit the pulse signal, which is used in the A / D coupler as a carrier for the Pulse-pause difference modulation is used. So that's it modulated and quasi-digitized output signal on the input pulse  synchronized; Multi-channel systems can be on one common clock can be synchronized and optimized. The PPDM process has three more that are essential for practical use Advantages:

• 1. Pre-reinforcements can usually be omitted.
• 2. Measuring range switches are to the digital signal range limited and therefore remain without influence on the Measurement result.
• 3. The creation of digital interfaces only causes little effort.

The basis for the A / D conversion is the input pulse with the period T ₀. Within T ₀ the PPDM circuit defines two successive phases T ₁ and T ₂, the difference T ₂- T ₁ proportional to the input voltage U _E and the sum of T ₁ + T ₂ is a constant, for. B. is equal to T ₀.

For the practical application of the circuit, it proves to be advantageous to define a further functional condition, namely the proportionality between T ₂- T ₁ and the period T ₀. This connection is realized electrically in that

• 1. a capacity is charged by a steady current ± i _E that is additionally charged during the phase T ₁ with a reference current i _K and that
• 2. a periodic discharge takes place during a period T ₂ by means of a negative reference current - i _K.

The following relationship applies to the basic function:

± i _E · T ₀ + i _K · T ₁ = i _K · T ₂

± i _E · T ₀ - i _K · (T ₂ - T ₁) = 0; with T ₀ = T ₁ + T ₂ applies:

± i _E · (T ₁ + T ₂) - i _K · (T ₂ - T ₁) = 0

Further functional advantages arise if an additional time phase T _{off is} inserted into the period T ₀, but this only affects the digital area, while the continuity of the analog control should continue to exist.

In order to maintain the basic relationships, the relationship between T ₁, T ₂, T _off and T lichen is defined as follows:

T ₁ ′ + T ₂ ′ = T ₀ - T _off

T ₁ ′ = T ₁ - T _off / 2, T ₂ ′ = T ₂ - T _off / 2

T ₂ ′ - T ₁ ′ = T ₂ - T ₁

This means that the charging and discharging times T ₁ and T ₂ change by absolutely the same amount compared to the basic function, while the difference between the two time values as the actual result of the implementation remains the same as that of the basic function.

The change in the charging and discharging times requires an adjustment of the charging and / or discharging current to values which ensure the charge balance during T ₀.

A variant of such an adjustment is defined as follows:

(± i _E + i _R ) · T ₀ = i _C · T ₂

(i _R and i _C are like i _K reference currents)

Electrically, the charge balance is realized via T ₀ by means of a control circuit in which the period of the input pulse T ₀ _{in is} compared as a setpoint with the phase T ₀ for charge compensation and the control deviation Δ T is fed back.

Since there is a proportionality between the residual voltage proportional to the charge equilibrium Δ Q and the control deviation Δ T , Δ T is detected indirectly via a voltage comparison. Similarly, the feedback of Δ T takes place indirectly via a charge proportional to Δ T and the voltage proportional to it (see diagram).

In the closed control loop, all occurring parameters, namely i _C , i _E , i _R and T ₀, are externally specified values. T ₁ and T ₂ regulate themselves as the only independent, but related variables with T ₀ accordingly to the setpoint.

The signal T ₀ _out used for the feedback fulfills the conditions placed on the A / D conversion at the beginning.

An embodiment of the Invention explained.

A block diagram is shown in FIG . The input pulse T ₀ _in has the period T ₀. The output pulse T ₀ _out has the period T ₀ and the phases T ₁ ', T ₂', T _off .

Fig. 2 shows the diagram with the control function of the analog / digital converter. The following applies: T ₀ ′ = T ₀ ± Δ T. T ₀ ′ is the actual value of the period. T ₀ ′ represents the not yet adjusted state.

Fig. 3 shows an application example DVM, 5 digits and with the counter SUPER-SUB-CUB. This SUPER-SUB-CUB is a meter from Red Lion Controls, Willow Springs Circle, RD5, York, Pa. 17 402. The resistance 2.4 M ^x is included in the front frame set.

Fig. 4 shows the timing diagram of the PPDM-01.

Claims (4)

1. Method for converting an analog input signal i _E into a digital output signal, the analog signal first being converted in a first conversion step into a pulse-pause-difference-modulated (PPDM) signal of the period T ₀ by combining the analog input signal with a first reference signal i _R over the period T ₀, which is predetermined as a control variable, is integrated and during a discharge phase T ₂, which either corresponds to the pause duration or the pulse duration, a second reference signal i _C is additionally integrated, for which the Condition applies i _C · T ₂ = (± i _E + i _R ) T ₀, and then the PPDM signal in a second conversion step by means of an up-down counter, which clock pulses of constant frequency depending on the front-back input of the counter supplied PPDM signal counts, is converted into the digital output signal. 2. The method according to claim 1, characterized in that the externally predetermined control variable T ₀ can contain information in the form of frequency modulation. 3. The method according to claim 1 or 2, characterized in that the period T ₀ from the pulse duration T ₁ ', the pause duration T ₂' and a constant dead time T _off , the difference T ₂'- T ₁ 'both proportional to Input signal i _E as well as the period T ₀. 4. The method according to any one of claims 1 to 3, characterized in that the reference current i _R is greater than the expected maximum input current i _Emax.