Principle and application of dual 8-bit multiplying digital-to-analog converter TCL7528

　　The TCL7528 is a dual, 8-bit digital-to-analog converter designed with separate on-chip data latches. Features include very tight DAC-to-DAC consistency. Data is transferred to either of the two DAC data latches via a common input. Control inputs DACA/DACB determine which DAC is loaded. The device's load cycle is similar to the write cycle of a random access memory, and it can easily interface with most general-purpose microprocessor buses and output ports. Segmenting high-order bits can minimize the flicker during the change period of the most significant bit, and the flicker pulse during this change period is the strongest.

　　The working power supply of TCL7528 is 5~15 V, and the power consumption is less than 15 mW. The 2- or 4-quadrant multiplication capability makes this device a good choice for many microprocessor-controlled gain setting and signal control applications. It can work in voltage mode, producing voltage output instead of current output.

1 Introduction to the chip 

1.1 Functional features

 ·Easy to interface with the microprocessor;
 ·On-chip data latch;
 ·Monotonicity within each A/D conversion range;
 ·Can interact with analog devices AD7528 and PMI PM7528 Replacement;
 ·Suitable for fast control signals including digital signal processing (DSP) applications interfaced with TMS320;
 ·Voltage mode (VOLTAGEMODE) operation;
 ·CMOS process manufacturing.

1.2 Functional block diagram

 : The functional block diagram is shown in Figure 1.

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1.3 Pin arrangement

　　The pin arrangement of TLC7528 is shown in Figure 2.

1.4 Timing diagram

　　The timing diagram of TLC7528 is shown in Figure 3.

2 Working principle

　　TLC7528 contains two identical 8-bit multiplying D/A converters DACA and DACB. Each DAC consists of an inverting R-2R ladder network, analog switches and data latches. Binary-weighted current switches between the DAC output and AGND, thus maintaining a constant current in each ladder branch, regardless of the switching state. Most applications only require the addition of an op amp and voltage reference. A simplified D/A circuit for DACA with all data inputs low is shown in Figure 4.

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　　TLC7528 interfaces with the microprocessor through the data bus and DACB control signals . When and are both low, the TCL7528 analog output (specified by the DACA/DACB control line) responds to activity on the DB0~DB7 data bus inputs. In this mode, the input latch is transparent and the input data directly affects the analog output. When the OR signal goes high, the data on the DB0~DB7 input terminals is latched until the OR signal goes low again. When  is high, data input is disabled regardless of the state of the WR signal.

3TLC7528 is used in programmable state variable filters.

　　TLC7528 is used in programmable state variable filters to provide low-pass, high-pass and band-pass outputs, and is suitable for applications that require microprocessor control of filter parameters.

　　As shown in Figure 5, DACA1 and DACB1 control the gain and Q value of the filter, while DACA2 and DACB2 control the cutoff frequency. For the cutoff frequency expression to be true, the DACA2 and DACB2 halves must track exactly. This is easily accomplished with the TLC7528.
The
　　programmable cutoff or center frequency range is 0~15kHz, and the Q range is 0.3~4.5. 　　Circuit equation: