THS4631: What is the simplest way to realize such an analog switch

Yatao Ling1

Part Number: THS4631

Dear TI colleague,

I want to realize the typical resettable integrator based on THS4631 as the Figure below.

But I have doubt about what is the simplest and cheapest way to realize the analog switch SW below? The 4631 is powered by +18V and -12V and its output is within 0V and +18V. The SW is controlled by a pin of FPGA (field programmable gate array). And I hope the turn-on and turn-off of SW should not be too slow. It is desired to be finished within, e.g., 100ns.

A signal MOSFET might not be suitable. Because FPGA pin's drive capability is always limited, e.g., below 10mA. Driving a MOS may be hard for FPGA pin.

Best regards

Yatao

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 317370 points

There are many suitable analog switches if negative voltages are not required.

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi Yatao,

keep an eye on charge injection :-)

Can you show a schematic with component values?

Kai

0 Yatao Ling1 over 5 years ago in reply to kai klaas69

Expert 1840 points

Dear Kai,

Glad to meet u again.

I want to thank you. With your previous help and assistance, I've succeeded to implement my experimental setup. With the setup, I've done many experiments and written an article for my PHD degree. The article has been accepted by a world-leading top journal of IEEE in my area.

You are definitely high-level, knowledgeable, kind, patient and can offer some valuable advice.

As to my current thread, to avoid complicated talk and not to occupy too much of your time, I just want to know if the simplest and cheapest way to implement such an analog switch is to use TI switches offered by Clemens, not by a MOSFET?

Best

Yatao

0 Clemens Ladisch over 5 years ago in reply to Yatao Ling1

Guru 317370 points

When using a MOSFET, the gate-to-source voltage must be high enough (or low enough for P-channel). When the signal to be switched can be anywhere between 0 and 18 V, you need to add some level shifting circuit, so it would be simpler to use a device that integrates the gate driver with the switch. (And an analog switch gives you two MOSFETs in a transmission gate.)

0 Yatao Ling1 over 5 years ago in reply to Clemens Ladisch

Expert 1840 points

Dear Clemens:

Thx for your patience to answer my question.

I get your point. But by saying "And an analog switch gives you two MOSFETs in a transmission gate.", you mean that signals can transmit in a bidirectional way?

Best

0 Clemens Ladisch over 5 years ago in reply to Yatao Ling1

Guru 317370 points

A single MOSFET would be able to handle bidirectional signals, but cannot handle signal voltages near the gate voltage. A transmission gate made of two MOSFETs allows you to handle signals with all voltages between the power rails.

0 Yatao Ling1 over 5 years ago in reply to Clemens Ladisch

Expert 1840 points

Hi Clemens,

Thx!

I hope my question is fine for you.

Best regards

0 kai klaas69 over 5 years ago in reply to Yatao Ling1

Guru 140200 points

Hi Yatao,

thank you for your warm words and congrats for your PHD degree :-)

Can you at least reveal the value of capacitance C1?

Kai

0 Yatao Ling1 over 5 years ago in reply to kai klaas69

Expert 1840 points

Hi Kai,

CI=680pF. RI=357R

As to the charge injection, the vE at the input, is periodic. First, vE is negative for some time, then positive for some time and this loop goes on and on.

Best regards

Yatao

0 kai klaas69 over 5 years ago in reply to Yatao Ling1

Guru 140200 points

Hi Yatao,

like this?

4857.yatao_ths4631.TSC

Kai

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THS4631: What is the simplest way to realize such an analog switch