ispPAC and its filter design

About ispPAC

Since 1992, Lattice, an American company, launched the In-System Programmability technology, adding a design and implementation method that is different from the traditional digital electronic system. At the end of 1999, Lattice launched the In-System Programmable Analog Circuit, which opened up a new way of thinking in analog circuit design and opened up a broader prospect for electronic design automation (EDA) technology.

Like the digital system programmable large-scale integrated circuit (ispLSI), it allows designers to use development software to design, modify, and simulate circuit characteristics on a computer, and finally download the design to the chip via a download cable.

Programmable devices in the system can achieve three basic functions: (1) signal adjustment; (2) signal processing (3) signal conversion. Signal adjustment mainly involves amplification, attenuation and filtering of signals. Signal processing involves summing, subtraction and integration of signals. Signal conversion involves converting digital signals into analog signals. Currently, three devices have been launched: ispPAC10, ispPAC20 and ispPAC80. Its development software is PAC-Designer from Lattice.

The programmable analog circuit in the system has the functions of amplifying, converting and filtering analog signals. It is achieved by interconnecting multiple functional blocks in the device and reconfiguring the circuit to adjust the gain, bandwidth and threshold of the circuit. The programming times can reach 10,000 times. Its internal structure and circuit are shown in Figures 1 and 2.

The structure of the IspPAC10 device consists of four basic unit circuits (PAC), an analog routing pool, a configuration memory, a reference voltage, an automatic correction unit, and an ISP interface. The device is powered by a single 5V power supply. The basic unit circuit is called a PAC block, which consists of two instrument amplifiers and an output amplifier, and is equipped with resistors and capacitors to form a true differential input/differential output basic unit circuit, as shown in Figure 2. The so-called true differential input/differential output means that each instrument amplifier has two input terminals, and the output of the output amplifier also has two output terminals. The input impedance of the circuit is 10 9, the common mode rejection ratio is 69dB, and the gain adjustment range is -10 to +10 times. The gain and characteristics of the circuit in the PAC block can be changed by programmable methods, and the device can be configured to various gains of 10 to 10,000 times. The capacitor CF in the output amplifier has 128 values ​​to choose from. The feedback resistor RF can be disconnected or connected. The basic units in the device can be interconnected through the analog routing pool to realize various circuit combinations.

Each PAC block can form a circuit independently or in cascade to realize complex analog circuit functions. For example, each PAC block works as an independent circuit. Figure 4 (b) shows four PAC blocks cascaded to form a complex circuit. The combination of basic unit circuits can be used to amplify, calculate second-order active filters and ladder filters without connecting resistors and capacitors to the outside of the device.

Filter Implementation

In a practical circuit system, its input signal is often affected by interference and other factors, and contains some unnecessary noise components, which should be attenuated to a small enough degree to separate the signal we need from the input signal source. In order to solve the above problem, we can use active filters to achieve it.

The following is an introduction to the design method of using in-system programmable analog devices to implement filters. Usually we can use three operational amplifiers to implement the circuit of the biquad function. The biquad function can implement various filter functions, such as low-pass, high-pass, band-pass, band-stop, etc. The expression of the biquad function T(s) is as follows, where m=1 or n=1 or 0:

Since the sensitivity of this circuit is quite low, the circuit is easy to adjust, and another notable feature of this circuit is that only a small number of components need to be added to realize various filter functions. Here we take the implementation of the low-pass function filter as an example to illustrate the entire design process. For example, the transfer function Tlp(s) of the low-pass filter is as follows:

After sorting, we get:

Taking b=k1k2, formula (3) can be rewritten as follows:

Its equivalent block diagram is shown in Figure 3:

It is not difficult to see from the functions in the block diagram that the system can be implemented using an inverter circuit, an integrator circuit, and a lossy integrator circuit. Substituting each operational amplifier circuit into the block diagram of Figure 3, the following implementation circuit can be obtained, as shown in Figure 4.

Now we no longer need to use resistors, capacitors, and op amps to build circuits and then debug the circuits. Now we can easily implement this circuit using programmable devices in the system. ispPAC10 can implement each functional block in the block diagram. The PAC block can sum or subtract two signals. k is the programmable gain. In the circuit, k11, k12, and k22 are set to +1, and k21 is set to -1. Therefore, a three-op amp biquad function circuit can be implemented with two PAC blocks. Use the schematic input method in the development software PAC-Designer to connect the two PAC blocks. The circuit is shown in Figure 5.

CF in the circuit is the feedback capacitor value, Re is the waiting resistor of the input op amp, and its value is 250KΩ. The outputs of the two PAC blocks are V01 and V02 respectively. Two expressions can be obtained, namely the bandpass function expression and the lowpass function expression.

The system's in-system programmable analog circuit development software PAC-Designer contains a macro unit specifically for filter design. According to our design requirements, we calculate the feedback capacitor, resistor, circuit gain and other parameters, and then perform circuit wiring and modify the corresponding parameter values ​​in the circuit diagram programming environment. The system design work is basically completed. In order to verify whether the design can meet the ideal requirements, there is also a simulator tool in the development software for simulating the amplitude-frequency and phase-frequency characteristics of the filter. If the simulation results meet the requirements, they can be downloaded directly to the ispPAC10 device via a download cable, and the entire design work is completed. Otherwise, modify the parameters until the simulation results are satisfactory.

Conclusion

It is extremely convenient to design analog filters with ispPAC-Designer. A PC can replace the usual oscilloscope, sweeper and breadboard, which greatly improves work efficiency. The system can be adjusted according to user needs without making any changes to the PCB board. It only needs to be redesigned and simulated in the PC through the software, and then downloaded to the chip. The whole work can be completed in a few hours.