# how to test comparator's propagation delay?

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#### wangwang96

##### Junior Member level 1 a comparator circuit ,how to simulate propagation delay by Hspice or orher simulation tools?
Thx!

#### paulux Given the fixed input waveform signal, e.g. full swing square wave input is applied to one input of comparator and another input is fixed voltage, then you can observe how long it takes to response.

#### sankudey

##### Full Member level 3 Hi,
The above post gives the method to be followed. I wand to add some points to that.

1.
Out of the two (or four if differential) input terminals, you give a fixed voltage (say Vr) to one of the terminal (say V+). This will work as reference to the comparator. Now apply a square wave to the other terminal (say V-).
-->If the value of Vr is some where between the ICMR of the comparator (excluding the extreme points), the comparator output sould change. Now, you have two points in the transient analysis (a) the time when the square wave (V-) crosses Vr and (b) when the comp. output changes its decission. The time difference between (a) and (b) should be the propagation delay.

2.
One important point must be kept in mind that the voltage levels of the V+ and V- are important. Because, to detect little voltage difference the comp. should take more time than it should take to detect higher voltage difference at V+ and V-. Keep this in mind. In my opinion instead a generic term "propagation delay", it is better to say "minimum propagation delay" or "maximu propagation delay".

Thanks,
sankudey

#### allan_guo

##### Junior Member level 3 sankudey said:
Hi,
The above post gives the method to be followed. I wand to add some points to that.

1.
Out of the two (or four if differential) input terminals, you give a fixed voltage (say Vr) to one of the terminal (say V+). This will work as reference to the comparator. Now apply a square wave to the other terminal (say V-).
-->If the value of Vr is some where between the ICMR of the comparator (excluding the extreme points), the comparator output sould change. Now, you have two points in the transient analysis (a) the time when the square wave (V-) crosses Vr and (b) when the comp. output changes its decission. The time difference between (a) and (b) should be the propagation delay.

2.
One important point must be kept in mind that the voltage levels of the V+ and V- are important. Because, to detect little voltage difference the comp. should take more time than it should take to detect higher voltage difference at V+ and V-. Keep this in mind. In my opinion instead a generic term "propagation delay", it is better to say "minimum propagation delay" or "maximu propagation delay".

Thanks,
sankudey
Hello sankudey: It 's right that the delta-Vab will determine the propagation delay ,but how can we test the minimun propagation delay and maxium one?If we test the min p-delay,if the difference between Va and Vb is small enough,the time will be very long, so how to determine the delta-Vab when we test the propagation delay?

#### sankudey

##### Full Member level 3 As much I can say is following...

For determining propagation delay you have to give a square wave in one input where as other one should be a DC. The difference in voltage is important for the magnitude of the propagation delay....those can be done in following manner....

1.
For minimum propagation delay-->
Put your reference(V+ terminal, DC) to (ICMR- + X) volt, Where ICMR- is the minimum common mode input voltage and X is the resolution of the comparator(minimum voltage the comparator can detect when running at its maximum clock frequency). Now you give a square wave in the V- terminal. The lower(zero) value of the square wave would be ICMR- and upper(one) value of the square wave would be ICMR+. By this test case, you can determine the minimum propagation delay.
=> you need to repeat the test case for the reverse condition, i.e., V+ at (ICMR+ -X) volt and the square wave accordingly. If the propagation delay for both of these are same (or almost equal) than its OK. Other wise the comparator operation depend on the common mode voltage and you may need to take the average of the above two prop. delay. I am not too sure of the previous line.

2. For maximum prop. delay-->
The V+ should be at (ICMR- + X) and square wave should be from ICMR+ to ICMR-. This is some times called overdrive test. This will give you maximum prop. delay.
=> agin you need to repeat the test for opposite polarity.

Hope, this will make you understamd the matter.

good luck!
sankudey

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