The LM7581 (a.k.a. AD7581) by Analog Devices (a.k.a. ADI) is a very interesting chip. ADI call it a 'Data Acquisition System', which means that this chip can measure analogue voltages, digitize them and pass them on to a microprocessor. I found a couple of these chips in a box of old ICs that someone gave me for free on a flea market, and according to ADI's website I'm very lucky since these chips sell for $26.57 each! I don't mind sacrificing one for microscopic examination however, so let's see what's inside :-)
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This is the top half of the chip. It's made in a CMOS process. All devices are clearly visible, and there at most a few hundred transistors. If you take a look at the datasheet, it's not too hard to figure out what the different parts are doing.
The four black squares at the top are a resistive divider. If you look carefully you can see thin lines inside them. These lines are the actual resistor, made of polysilicon. The black square in the top-right corner is a bank of resistors, one for each analogue input. This chip has eight analogue inputs, but it has only one A/D converter, so it needs a switch to select one of the inputs. This kind of switch is called a multiplexer, or 'mux' for short, which we see at the right. At the bottom we see the memory cells.
The bottom half shows the rest of the memory. There is a total of 8x8, or 64 bits.
A close-up of the input resistors. There are a total of nine resistors, one for each input and one for the offset calibration voltage (pin 1, see datasheet). The text at the top and the logo on the right tell us that we're looking at the right chip :-)
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A close-up of the multiplexer. Each analogue input is connected to one side of a rather large transistor. All the outputs of the transistors are connected together, and the digital circuit decides which one to connect by setting a high signal on the corresponding transistor's gate. This particular multiplexer has two outputs, meaning that each signal can be directed to output A, to output B, or be blocked altogether.
Here is the control logic that decides where to send the data. On the left is the memory area.
The memory cells in detail. Eighteen cells are shown here, each of which contains one bit of information. Standard CMOS RAM cells are made of six transistors.
A closer look at the resistive divider we saw earlier. Below the resistors is again a multiplexer, but this time not all transistors are the same size: they decrease in size from left to right. This is because the left transistor is connected to a small resistor, and will therefore have to supply a larger current. Because the resistors become bigger as we move to the right, the currents become smaller and hence the transistors become smaller as well. At the top we also see three (somewhat dirty) test structures, two of which are labelled 'P' and 'N', for PMOS and NMOS transistors respectively.
ADI is friendly enough to indicate pin 1 to us :-)