Circuit Design

The acceptable input range for all of the chips that we were looking at is -0.3V to 4.6 V. However, the analog input signals go from –3 V to 3V. Since we are only interested in the positive part of the inputs we can rectify the signals to get rid of the negative part. To do this, BAS16XV2T1 diodes were used. They were chosen because of their extremely small size and cost.

The outputs of the 2801 are digital signals, but the final outputs need to be analog. To handle this problem, the digital signals were sent through DACs, digital-to-analog converters. TI makes DACs that can handle up to 8 circuits. However, these chips require both an analog supply voltage and a digital supply voltage. Since there is not access to a digital supply where the circuits are going to be used, the design is limited to a maximum of 4 circuits per DAC. Similar to the DSP chip, the most important aspect for choosing a DAC is the board area and cost. As you can see from this table, the DAC6574 is the best DAC in terms of cost per circuit and board area per circuit.

Another issue that we ran into concerns the analog output from the DAC. The desired analog output ranges from 0-7.5 V. TI does make DACs that can handle this output range, but there are some tradeoffs. The DAC7731 is one of these capable of this range, but as you can see it is similar in size to the entire analog circuit and costs almost twice as much.

To solve this problem with the output, an operational amplifier after the DAC was used. The TLV2774 is being used because it contains 4 amplifiers per chip and is very small and cheap. Since the output range is 0-5 V and it needs to be 0-7.5 V, a gain of 1.5 for the amplifier stage is needed. This was done by placing a1k resistor between the output and the negative input and a 2k resistor between the negative input and ground.