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LM331: LM 331 Datasheet Help - Possible Error

Part Number: LM331
Other Parts Discussed in Thread: OPA197, LF411

Good Day

I have been tryiing to understand the datasheet of the LM 331 specifically the Voltage to frequency converters. Several question have occurred to me as well as confusion.

1) On page 12, Figure 14 (Figure 14. Simple Stand-Alone V-to-F Converter with ±0.03% Typical Linearity (f = 10 Hz to 11 kHz) seems to be in conflict with the ranges given in section 9.2.1.1 - Design Requirements. The figure states Vin is 10V full scale and Fout as 10kHz. but yet in the design requirements it is stated that " For this example, the system requirements are 0.05% linearity over an output frequency range of 10 Hz to 4 kHz with an input voltage range of 25 mV to 12.5 V. " This seems to be in conflict with each other.

2) When i use the formula ƒOUT = (VIN / 2.09 V) × (RS / RL) × (1 / RtCt). By substituting the values from the diagram I get an Fout value of 10554,46102 Hz for a Vin of 12.5 V. This is not the 4kHz value which was required.

3) Does the supply Voltage have any effect on the circuit? so any Vs voltage within the range of  4 to 40 V shall produce near identical Frequencies?

4) The Rs value should always be atleast 12k to 16k  to ensure that not too much current gets draw from the internal circuit. therefore to modify your frequency output only Rt and RL an be manipulated and should be the same?

5) The 5K adjust for Rs is that to cater for the component intolerances?

6) The current output from Pin 3 is relatively small (typically mA) therefore if more current is needed it should be used with a transistor or a buffer amplifier?

7) What is the 22K offset adjust? Is it a 22K potentiometer? Is it important?

8) My objective is to convert a 1-5 Volts signal into a proportional frequency. Then pass it through an optoisolator. Then convert it back to a voltage, using a frequency to voltage converter on the other end. Is this actually possible using the Lm 331 for both V-F and F-V conversions? Will It be relatively accurate?

9) Is the LF411CP  op amp a good replacement for the LF411A op amp found in section 9.2.2 Precision V-To-F Converter. Page 14 figure 16 of the datasheet?

 

I thank you in advance for any help that I may receive.

  • Hello Mohammad,

    I don't know the history of the LM331 datasheet but it is certainly possible that errors have made their way into it over the years and revisions. I do agree with you that things don't agree in some cases. I'll do my best to answer you concerns, but the information available about this device is limited to what can be found online, or TI.com. Regarding your questions:

    1) On page 12, Figure 14 (Figure 14. Simple Stand-Alone V-to-F Converter with ±0.03% Typical Linearity (f = 10 Hz to 11 kHz) seems to be in conflict with the ranges given in section 9.2.1.1 - Design Requirements. The figure states Vin is 10V full scale and Fout as 10kHz. but yet in the design requirements it is stated that " For this example, the system requirements are 0.05% linearity over an output frequency range of 10 Hz to 4 kHz with an input voltage range of 25 mV to 12.5 V. " This seems to be in conflict with each other.

    That statement about the typical linearity for f = 10 Hz to 11 kHz is the same as what is listed in the 7.5 Electrical Characteristics table. I am not sure what it was restated with the Figure 14 example, but possibly just a reminder of what linearity performance can be achieved over that frequency range.

    The F-to-V equation is correct. I found an LM331 applications note that was revised in 2013 and it backs up.

    www.ti.com/.../snoa735b.pdf

    2) When i use the formula ƒOUT = (VIN / 2.09 V) × (RS / RL) × (1 / RtCt). By substituting the values from the diagram I get an Fout value of 10554,46102 Hz for a Vin of 12.5 V. This is not the 4kHz value which was required.

    If I plugged the numbers into the calculator correctly I get about 4.3 Hz output for a 25 mV input, and 2.16 kHz for a 12.5 V input. That doesn't coincide with the 10 Hz to 4 kHz. I used Rs = 14 k, RL = 100 k, Rt = 6.8 k, Ct = 10 nF.

    3) Does the supply Voltage have any effect on the circuit? so any Vs voltage within the range of  4 to 40 V shall produce near identical Frequencies?

    The Electrical Characteristics table states: All specifications apply in the circuit of Figure 16, with 4.0 V ≤ VS ≤ 40 V, TA = 25°C, unless otherwise specified. Certainly there will be some minor variations in frequency with supply voltage.

    4) The Rs value should always be atleast 12k to 16k  to ensure that not too much current gets draw from the internal circuit. therefore to modify your frequency output only Rt and RL an be manipulated and should be the same?

    The optimum value for Rs is 14 k. Manipulate Rt and RL to meet the frequency output requirements.

    5) The 5K adjust for Rs is that to cater for the component intolerances?

    It certainly allows for linear frequency adjustment and would offset errors due to component tolerances.

    6) The current output from Pin 3 is relatively small (typically mA) therefore if more current is needed it should be used with a transistor or a buffer amplifier?

    Pin 3 is an open-collector output which requires a pull up to a positive supply. The Electrical Characteristics table lists: VSAT I = 3.2 mA (2 TTL Loads), 0.1 to 0.4 V. That is a fair amount of current. You can buffer it with a digital gate if more drive is needed.

    7) What is the 22K offset adjust? Is it a 22K potentiometer? Is it important?

    All analog circuits have some degree of voltage offset. Add the offset potentiometer if the offset needs to be zeroed in your application, but do note that it will change with temperature changes.

    8) My objective is to convert a 1-5 Volts signal into a proportional frequency. Then pass it through an optoisolator. Then convert it back to a voltage, using a frequency to voltage converter on the other end. Is this actually possible using the Lm 331 for both V-F and F-V conversions? Will It be relatively accurate?

    The LM331 can be used for V-to-F and F-to-V conversion as the datasheet indicates. It will be as accurate in each case as the datasheet states.

    9) Is the LF411CP  op amp a good replacement for the LF411A op amp found in section 9.2.2 Precision V-To-F Converter. Page 14 figure 16 of the datasheet?

    The LF411 is an obsolete op amp. I suggest using a newer, higher performance op amp such as the OPA197:

    www.ti.com/.../opa197.pdf

    Regards, Thomas

    Precision Amplifiers Applications Engineering