What are AC and DC Charecteristics for a chip?

Question

I want to understand how DC and AC charecteristics are determined for a chip. How are they obtained? How are they useful in implementing the solution. What do they represent?

Answer 1

The DC characteristics tend to represent mininum, nominal and absolute values for supply voltage, output current drive per pin or package, and that sort of thing. If you look carefully at a spec sheet, these items will be spelled out separately from the others.

AC characteristics have to do with things like switching speed (e.g., output change with response to input, often called "propagation delay"), slew rates, input capacitance (helps describe the load your input creates for other parts) and other factors having to do with the way signal move into, through and out of the part. Here's my old axiom: There's no such thing as digital -- it's all just funny looking analog. Nowhere is this more true than in high speed logic design. If you don't treat these signals carefully as one would in an analog design, they can easily come back and bite you.

Many spec sheets will show a sample test circuit that describes an environment that could be created to test the AC signal characteristics of a chip.

As for usefulness --

You would want to pay attention to the "speed" of a chip compared to the requirements of your design. In addition to the voltage you plan to use in your design, switching speed may well play a big part in the decision you make about which "family" of logic you wish to use. For example, if you need for a process to operate at 2ns cycles (500MHz), selecting a 74LS family part (where the maximum switching speed might be on the order of 5~6ns) wouldn't be a good decision. The part wouldn't be able to change states fast enough to be useful in that circuit.

The manufacturer represents that a given chip will behave within the limits specified on the spec sheet. For example, propagation delay will usually have a "typical" and a worst case "maximum" value. This will be decided by the number of gates and the construction of a given family of logic. The manufacturer can type test a chip for such specifications by attaching an appropriate AC signal source (clocks and the like) to the front end of the chip, and then observe that the output shifts within the specified limits. Whether the simulation circuits for this are actually shown on the spec sheet varies with the manufacturer.

Answer 2

DC characteristics are things like:

- Logic voltages for the inputs and outputs, for both the 'high' and 'low' states.
- Current source and sink capability for the outputs.
- Current source values for the inputs (this helps to determine fanout for the previous device in the logic chain).
- Total current draw from the power supply.

AC characteristics are things like:
- Propagation delay time (input to output)
- Setup and Hold time (for flip-flops)
- Maximum frequency of operation
- Minimum frequency of operation (for some devices that have dymamic RAM type registers)
- Propagation time for control lines (output enables, resets, etc.)


These values are determined by the manufacturer of the chip, mostly by taking samples off the production line (from some of the very first items 'off' the line) and actually measuring voltages, currents, and delay times. All these values are published in the datasheet for the part.

In order to properly design a logic circuit you must know all of these parameters.

For example, you could not connect a TTL part output to a 4000 CMOS input -- the minimum output high of a TTL part is only 2.7 volts, while the minimum 'high' voltage of a 4000 CMOS part is 2.75 Volts. 0.05 volts is *way* too marginal to be a good design.

Example 2: You cannot connect an LSTTL flip-flop to a Xilinx FPGA running at 250 MHz, because the LSTTL flip-flop has a max. frequency of less than 50 MHz. It just wouldn't work.

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Answer 3

All chips operate with a DC power source. From there you can break them down into analog or digital (and a mix of both called mixed signal chips).

The design process for analog chips is quite different from digital chips. In digital design the problem is managing scale (and power dissipation) but the devices are either on or off. In analog design you need more stable devices (not the bleeding edge) and lots of know how to get repeatable production results. The devices in an analog design can be anywhere between on and off and they need to be wherever predictably.

Answer 4

All the chips just work on DC current. The AC sometimes are a clock frequency to be counted down as a time standard .These are used a lot for clocks and freq dividers.

Answer 5

AC means some signals that change between two number.
DC means a signal that all the time has a quantity.
whit a signal generator , we see AC &DC signals.if you press AC button you can see AC. DC is like a line.bute AC is changable.