74VHC4020FT counter IC is not operating to spec above 3 V supply voltage, suggestions?

Thread Starter

gepeg

Joined Jan 11, 2022
1
Issue: I'm noticing that when feeding a square wave to a 74VHC4020FT IC at a reasonable amplitude and frequency (amplitude: 0-3V or 0-5V, frequency: tried 1 kHz, 100 kHz, and 80 MHz) , the output from Q4 is not what I expect and strongly depends on the supply voltage. The Q4 output should divide the signal by 16, which occurs for a supply voltage between 2 V to ~3 V. Above this, there is a ~0.5 V window between ~3V and ~3.5V where the Q4 output divides by eight. Above ~3.5V the output is the supply voltage. I have tried this on several chips and measured the same behavior. Any advice would be welcome!

Manual: The chip should operate with a supply voltage (Vcc) between 2-5.5 V. The input voltage should be between 0-5.5 V (independent of the supply). The high level input voltage and low level input voltage depend on Vcc. For a Vcc of 2V, the high is above 1.5 V and the low below 0.5 V. For a Vcc between 3-5.5V, the high is above 0.7*Vcc and the low is below 0.3*Vcc.

Context: I grounded the CLR and GND pins, attached CK to a function generator, and Vcc to the power supply. The chip is soldered to a TSOOP-16 adapter and wires connect the ports to BNCs. The chip does not draw significant current.
 

eetech00

Joined Jun 8, 2013
3,149
Issue: I'm noticing that when feeding a square wave to a 74VHC4020FT IC at a reasonable amplitude and frequency (amplitude: 0-3V or 0-5V, frequency: tried 1 kHz, 100 kHz, and 80 MHz) , the output from Q4 is not what I expect and strongly depends on the supply voltage. The Q4 output should divide the signal by 16, which occurs for a supply voltage between 2 V to ~3 V. Above this, there is a ~0.5 V window between ~3V and ~3.5V where the Q4 output divides by eight. Above ~3.5V the output is the supply voltage. I have tried this on several chips and measured the same behavior. Any advice would be welcome!

Manual: The chip should operate with a supply voltage (Vcc) between 2-5.5 V. The input voltage should be between 0-5.5 V (independent of the supply). The high level input voltage and low level input voltage depend on Vcc. For a Vcc of 2V, the high is above 1.5 V and the low below 0.5 V. For a Vcc between 3-5.5V, the high is above 0.7*Vcc and the low is below 0.3*Vcc.

Context: I grounded the CLR and GND pins, attached CK to a function generator, and Vcc to the power supply. The chip is soldered to a TSOOP-16 adapter and wires connect the ports to BNCs. The chip does not draw significant current.
The logic levels are not independent of the supply. The data sheet clearly shows that.

Have you installed bypass capacitors across the supply pins of each IC?
Does the power supply have a filter cap?
Is the square wave glitch free? The device will increment on the falling edge...
 

crutschow

Joined Mar 14, 2008
29,779
As eetech00 noted, the input clock drive voltage must vary with the supply voltage.
Typically it is set to the same value as the supply.
 

Papabravo

Joined Feb 24, 2006
18,408
Issue: I'm noticing that when feeding a square wave to a 74VHC4020FT IC at a reasonable amplitude and frequency (amplitude: 0-3V or 0-5V, frequency: tried 1 kHz, 100 kHz, and 80 MHz) , the output from Q4 is not what I expect and strongly depends on the supply voltage. The Q4 output should divide the signal by 16, which occurs for a supply voltage between 2 V to ~3 V. Above this, there is a ~0.5 V window between ~3V and ~3.5V where the Q4 output divides by eight. Above ~3.5V the output is the supply voltage. I have tried this on several chips and measured the same behavior. Any advice would be welcome!

Manual: The chip should operate with a supply voltage (Vcc) between 2-5.5 V. The input voltage should be between 0-5.5 V (independent of the supply). The high level input voltage and low level input voltage depend on Vcc. For a Vcc of 2V, the high is above 1.5 V and the low below 0.5 V. For a Vcc between 3-5.5V, the high is above 0.7*Vcc and the low is below 0.3*Vcc.

Context: I grounded the CLR and GND pins, attached CK to a function generator, and Vcc to the power supply. The chip is soldered to a TSOOP-16 adapter and wires connect the ports to BNCs. The chip does not draw significant current.
As should be apparent, after careful consideration of what the datasheet is saying, both the output level AND the input threshold(s) are a function of the the supply voltage. Every time you change VCC, you need to reset the generator and the output detector to account for the change. That said, most people pick a VCC for a system and go with that single value, they don't go changing things on the fly. That would, IMHO, be nonsensical. Your mileage may vary.
 
Last edited:

BobTPH

Joined Jun 5, 2013
4,897
Did you connect the signal generator ground to the circuit ground? A 0-5V sugnal should certainly trigger the chip when supplied by 5V. Are you saying it dies not?

bob
 
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