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# TPL8002-25: RG RF and RSW value if not to use operational amplifier

Part Number: TPL8002-25

Hello,

I have some trouble about tpl8002-025, the chip of parallel input function is what I need.

I have connected VCC/VEE  with +- 3.5V,  Pins 10-15 are hooked with 3.5V( for test), Pins 9 and 16 are hooked with GND.I wonder know how to measure the value of RG RF and RSW.I I just wanna output the resistance, and not use in operational amplifier circuit. How get I use it as a resistance.

I want to use this chip to control LED brightness Dynamically,  which is high frequency.

Best regards

Kailyn

• Hi Kailyn,

The series wiper resistance for each wiper tap terminal, RSW, is set to 420Ω. RG and RF are given in the Switch Truth Table (Table 1) in the datasheet. If pins 10-15 are connected to 3.5V for test then RG will be 161 and RF will be 2339Ω.

In this diagram, VIN is applied to RG, GND is applied to RF and VOUT is on RSW. RG and RF create a voltage divider: VOUT= VIN*RF/(RG+EF)

Best,

Katlynne Jones

• Hi Katlynne，

Thanks for your answer sincerely. But if I want to use only the value of RF in series to a certain circuit, can it be achieved? I hope it can be used as a sliding rheostat for me.

Best regards

Kailyn

• Hi Kailyn,

Sorry for misunderstanding your question. Yes you may just use RF in the circuit. To do this, leave the RG pin floating and connect the RF and RSW pins to your circuit. The 420Ω series resistance of RSW is fixed so it will be added to the values given in the table for RF. This means the minimum resistance between the RF and RSW terminals will be 568Ω and the maximum resistance will be 2759Ω.

Best,

Katlynne Jones

• Ok, I understand your explanation. And can I add a resistance of the external circuit and  connect it parallel with the RF? If I need a smaller resistance value.

• Hi Kuijun,

Yes you may add a parallel resistance. The parallel resistance, RP will be in parallel with the sum of RF and RSW. The figure below shows a simplified diagram:

Best,

Katlynne Jones

• Thanks a lot ! I’m really appreciate it!

• Hi Kuijun and Kailyn,

Not a problem. Please let me know if you have any additional questions.

Best,

Katlynne Jones

• I found that after connecting resistors in parallel, the adjustable range is very small. If I want to get the resistance value range of 30-260, is there a way to do it?
• I used tpl8002-25 in parallel with a resistor to test its high-frequency performance, output a square wave of 8MHz or lower to the F interface (two different resistor dividers), and then measured the voltage across the parallel resistor. Logically, the voltage at both ends should also have two square waves with the same frequency, but the waveform effect is bad.

• Hi Kuijun,

Can you share a schematic of your setup and a screenshot of the the scope shot of the input and output waveform you are seeing.

Thank you,
Katlynne

• I connect VDD VEE with 3V, -3V voltage，Give F a square wave with a different frequency, 50KHz、500KHz、1MHz.According to the test below, the measured voltage value is no problem, When the frequency is high, the voltage value will be wrong, and the waveform page will be bad

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• I found that after connecting resistors in parallel, the adjustable range is very small. If I want to get the resistance value range of 30-260, is there a way to do it?

• Hi Kuijun,

The 8MHz bandwidth spec given in the datasheet is for a voltage being applied to one of the resistor terminals (seen in the figure below), not the digital control pins. There is no spec for the output response time due to a change on a digital control pin. The high frequency square wave you are trying to input on the control pin is likely exceeding that spec which is why you see the bad response. In the first scope shot, it looks like it takes about 5us for the resistance to reach the desired value, this would lead to about a 200kHz waveform. In your 500kHz waveform you can see that the output square wave does not have enough time to reach the max value like in 100kHz waveform.

I don't see a way to get the 30-260 range due to the formula for parallel resistance values.

Best,

Katlynne Jones

• So in this case, the high frequency can only almost reach 200KHz, right?

• Hi Kuijun,

That is what I am concluding based on the scope shots you have sent me. The device doesn't have a spec for this implementation. You can go past 200kHz but you will start seeing the degraded waveforms like you shared due to the device not being able to respond to the interface controls fast enough.

Best,

Katlynne Jones