MULTI-CHANNEL DIGITAL-ANALOG SYSTEM BASED ON CURRENT-CURRENT CONVERTERS

An approach to building a multi-channel digital-analog system is proposed, in which, unlike the known ones, a code-current converter, a controlled current generator, and also a current communication block are used. For a given accuracy, this saves on the analog system equipment. It has been shown that the proposed principle of building a controlled current source in the form of a highly linear push-pull amplifier – current scalar on bipolar transistors with a grounded load – has a high output resistance and wide bandwidth, which allows the use of current switching to implement the multi-channel system mode.


Introduction
Many electronic systems [2,6,7,11], such as programmable power supplies, signal distribution systems with a DAC in each information channel, multi-channel information transmission systems with sampling and storage devices with an analog multiplexer, use groups of digital-to-analog converters. At the same time, in some cases, it is advisable to use one DAC with multiplexer and output devices to capture or buffer the output signal. First of all, this refers to time distribution systems based on a single DAC with multiplexer and output devices for capturing or buffering the output signal.
It should be noted that in the case of using multiple DACs in the above systems, a number of important features need to be taken into account. First, each DAC has its own individual static errors and, if it is necessary to calibrate them, the system that implements this is complicated. Second, despite the ability to provide the required performance by the group DAC, it increases the power consumption and requires additional digital equipment.
On the other hand, the use of a single DAC requires an increase in its speed compared to the speed of any of the DACs in a group, as well as a high speed multi-channel output signal switch. A promising way to meet these requirements is the current principle in making these devices. Thus, the DAC must be made in the form of a code-current converter (CCC), the output buffer of the CCC is based on a controlled current generator (CCG), and a multi-channel analog switch is based on high-speed diode switches.
However, this approach is somewhat new, especially in the construction of CCGs, and it has not been sufficiently considered in the scientific and technical literature, so the topic of the article on the construction of a multi-channel digital-analog system on a single code-converter and controlled generator is urgent.
Research methodsproviding increased speed of high-line multichannel code-analog conversion based on the current principle, in particular, using a controlled current generator.

Research objectives
The authors performed an assessment of the characteristics of various types of the equipment, in particular, their technical data, characteristic features, advantages and disadvantages: 1. to offer a generalized block diagram of a multichannel digitalanalog system based on a single code-current converter and a controlled current generator; 2. to offer and analyze the principle of construction of a controlled current generator in the form of a highly linear push-pull amplifiercurrent scalar (PPACS) on bipolar transistors with a grounded load; 3. to estimate the static errors (zero offset and linearity) of the PPACS, as well as its speed, in particular the amplitudefrequency characteristic by computer simulation; 4. to consider the possibility of calibrating the error of the zero offset current ( ZO.0 I ) of the proposed multi-channel digitalanalog system, by introducing corrections presented in digital form; 5. to provide practical recommendations for the practical implementation and application of the proposed multi-channel digital-analog system.

Solving research problems
Analyzing the purpose of the research, as well as the set of functions to be implemented, it is possible to heuristically synthesize the system as the block circuit shown in Figure 1. This circuit contains a code-current converter (CCC), controlled current generator (CCG), current switch unit (CSU), digital code adder (DCA), table of zero offset channel error correction codes, address bus (AB) of channel selection, set of output devices (SOD) to which currents I 1 , I 2 , …, I k are supplied, and control block (CB), which ensures the functioning of the digital-analog system.
The system works in two modes. In the first mode, the zero offset errors of the source devices are determined. To do this, the DCA input receives zero codes   . The first mode is then completed and the system is ready to perform the main function. In the second mode, which is the main one, the work is done as follows. The i-th number of the required channel is selected by the CB command, and the converted code The generated i I is sent to the controlled current generator. It has the following requirements: high output resistance, as well as a wide bandwidth [9]. This is due to the specifics of the key current elements. Figure 2 shows a block circuit of the CCG and the current switch unit. Moreover, the first should be implemented in the form of a high-line push-pull DC amplifier (PPDCA), the schematic of which is considered in [1].
To ensure high output resistance Let us assume that the PPDCA is built according to the scheme shown in Figure 2 It should be noted that expression (2) will be valid provided that  It is known from the theory of transistor amplifier circuits in the case of using a negative connection with current removal [10] that this leads to an increase in the output resistance. In our case, we have:   The graphs show that the output resistance decreases slightly when the direction out I changes from direct to inverse.
The dependence out R as well as the linearity errors on tі C are given in the following table. The change out R is explained by the fact that in the range 1 mA 1 mA out I    at the outputs CM1 and CM2, the balance mode changes, during which there is a redistribution of collector currents between the p-n-p and n-p-n transistors, which have a significant difference between current gain n  and p  , and also different collector resistances. The CCG output is connected to the bridge diode keys (BDK) of the current switch unit. The BDK operating points are set by operating current generators It should also be noted that the application of the principle of current amplification allows us to achieve the maximum speed of the CCG, which is determined by the cutoff frequencies of bipolar transistors. So the bandwidth of a single gain PPDCA when 100 L R  reaches ~1.6 GHz. This can be seen from the amplitude-frequency characteristic of the device, the graph of which is shown in Figure 4. It is necessary to look at the recommendations for the use of types of output devices that are loads for the CCG and CSU, as well as this DA system. To maintain high speed and minimal linearity errors, it is desirable that the input supports out R of the output devices are low; not more than hundreds of ohms. This is easy to achieve in current-voltage converters built on operational amplifiers, as well as sampling devicesstorage of integrated type [3,4,5]. In some cases, the load can be the control winding of the stepper motor in the tracking codethe angle of rotation of the shaft [8].

Conclusions
1) An approach to building a multi-channel digital-analog system is proposed, in which, unlike the known ones, there is a codecurrent converter, a controlled current generator, and a current communication block. For a given accuracy, this saves on the analog system equipment.
2) The proposed principle of construction of a controlled current generator in the form of a highly linear push-pull amplifiercurrent scalar based on bipolar transistors with a grounded load has been analyzed. It has been shown that this device has a high output resistance and a wide bandwidth, which allows current switching to be used to implement a multi-channel system mode.