Discrete chopper switch

Hi CVictor914,

It looks like you want to create a discrete chopper amplifier, which was typical in the last century, because there were no such integrated circuits. I'm correct?

Regards,

Kirill

Hi KirV,

Thank you for your reply.

Acually, chopper amplifier is not my target. I just want to use a chopper to realize the function of multiplication. For clarification,  I have an analog input signal and I want to multiply it with a sequence of 1 and 0. And the output is connected to a differential amplifier.

I know it's hard to find a discrete chopper switch. Do you have any other suggestions?

Thanks.

Victor

Hi KirV,

Thank you so much for your help.

My idea is shown in the first figure below. I use a chopper (principle shown in the second figure) to multiply analog input voltage with +1 or -1 according to the control signal, say PRBS. And then, the output of chopper goes through a differential integrator. 

I am searching for a device which can achieve the function of chopper above (4 switches and controlled by a logic signal). So far, https://www.electroschematics.com/4066-ic-datasheet/ 4066 is a choice. Do you have some advice?

Furthermore, because here is the group for amplifier, I also wonder which amplifers fit for my differential integrator. 

http://www.ti.com/product/ACF2101/technicaldocuments This is the only integrated integrator chip I found. Do you know any amplifer from ADI can be easily implemented as an integrator with reset switches? It will be very appreciated!

Victor

Sorry KirV, I wrote a lot and used two figure to illustrate my idea. But my reply was determined to be SPAM. So sad. 

Anyway, thank you bro!

Hi Victor,

It's weird, I've never seen anything like it. You can try to attach ZIP archive to the message.

Hi KirV,

Thank you so much for your reply and patience. I am trying to edit it again.

My basic idea is shown as above. I use a chopper, whose principle is shown in below (composed of four switches and they are controlled by a binary signal), to multiply the input analog signal with +1 or 1 according to the bit of PRBS (1 or 0). Then the signal goes through an active RC integrator.

So I want to find a chopper device as above. However, when I search "chopper" in ADI product list, it only shows chopper-stabilized amplifer, which is not my target. I only want "four switches". Do you have any advice for the implementation of chopper?

Furthermore, I also want some advice for the implementation of integrator. The only integrated integrator I found is TI's ACF2101 and it's a little bit old design. So could I use some devices from ADI such as the amplifer to achieve it? Are there switches used as reset switches in my design? Due to a huge amount of amplifer in ADI's product list, I seached something but they don't fit for me. If you know some example designs from datasheets, please let me know. It will be very appreciated!

Victor

Hi Victor,

Now I understand your idea. Those devices that I recommended above seem to be perfectly suitable for a chopper. I very much doubt that using four switches as an alternative will get you anything better.

Why are ADI fully differential amplifiers not suitable for your application? What are your requirements for the circuit-supply voltage, permissible error, values of capacitors, speed? I may be able to advise you on suitable reset switches, but I need a little more information.

Regards,

Kirill

I am not familiar with integrated circuits that are called choppers or at least positioned as specially designed for this role. No wonder you couldn't find something like that with a search.

My appeal works...

Hi KirV,

Thank you so much for your generous help.

I read carefully the datasheets of LTC1043 and 6943 you recommended. To my understanding, the control clock signal of these two chips are generated internally using pin16 (in 1043) and pin14 (in 6943). Their leakage currents are so low and CMRR are perfect.

However, for my idea, I need a control signal, say PRBS in the above diagram, that is not a clock signal (say, duty cycle=50%). Its high and low levels are random. So it needs an external control signal. Do you know some devices suitable for my requirement? So far, my choice is a 2-bit bus exchange switch  https://www.onsemi.com/products/interfaces/analog-switches/7wb383. But it is actually used for digital signal. The leakage current is large. If there are analog swiches whose control signal can be given externally, they would be better.

For the integrator, I have noticed you replied my another thread, so I will disscuss with you in there.

Best regards,

Victor

Furthermore, the charge injection of switch is also a significant factor in my design. LTC1043 and 6943 surprisingly measure this parameter and have low charge injection. They provide break-before-make action (non-overlapping) as well. I can't find any these measurement data from digital switch datasheets.

If they can be controlled by the external random digital signal, I would say they are perfect options. If there is possibility that they can be used as I want but I can't come up with?

Finally, I don't know if you notice. My reply last night which I said it was determined as SPAM came back after my appeal. Yeah!

Hi Victor,

Yes I noticed almost the same thing I had already read, but dated December 9, which confused me a bit. But it's great that everything worked!

These devices are specifically designed for precision analog devices, and are used, in particular, in the most precision DMM in the world. They can be controlled by an external signal

It seems to me that when using a non-periodic signal, there should be no big problems if the durations of your signal allow the internal circuit to switch reliably

Hi Victor,

Yes I noticed almost the same thing I had already read, but dated December 9, which confused me a bit. But it's great that everything worked!

These devices are specifically designed for precision analog devices, and are used, in particular, in the most precision DMM in the world. They can be controlled by an external signal

It seems to me that when using a non-periodic signal, there should be no big problems if the durations of your signal allow the internal circuit to switch reliably

Hi KirV,

You are totally right!

I missed something before and now I know how to use them as a chopper. They are perfect choices.

By the way, did you use some single transistor switches? I know it will be crazy if I use 32 single switches on PCB to make 8 choppers I need. But if you know some high-performance analog switches, it would be better. I can use them as alternatives or use them to compare with LTC1043 and 6943.

Thanks a lot for your help!

Victor

Hi Victor,

Note how LTC1043 and 6943 are used in conjunction with capacitors - only the difference of the injected charges can create a voltage on the capacitor. Even a significant but identical injected charge applied to both capacitor plates does not affect performance.

The same unique property has a differential integrator - if the same charge is injected into both inputs, it will cause a common-mode shift of the output voltages, which is not a useful signal. Only the difference of the injected charges matters.

I find it difficult to advise you any switch. The difference of the injected charges is important. You can make individual selection of components or add a final adjustment of this parameter using external components. But it's not the best choice:)

Even more difficult is that the integrator has a small input resistance and the internal resistance of the switch can make an error, in addition, this resistance is nonlinear. What is the value of the input resistors and input currents of your integrator? This is important for selection

Regards,

Kirill

Hi Victor,

The product tables are sorted by parameters, and I tried to select the parameters important to you here

www.analog.com/.../10624

It is a simple interface, suitable contact configuration and minimal injected charge

I think I know why your message was marked as spam - you gave links to third-party resources and products that are prohibited by the forum rules.

Regards,

Kirill

Hi Kirill,

Thanks for your really helpful advice!

This is my first thread in this forum and I will obey the forum rules. Thanks for noticing me. And so lucky to disscuss with you and learn many stuff from you.

This chopper is for a function prototype PCB. Optimization will be at next stage. I think I have already known which switch I should use.

Thank you.

Victor

Thank you Victor!

By the way, which signal goes through the chopper-is it current or voltage? Does the signal source have a high or low internal resistance?

Regards,

Kirill

Hi Kirill,

In my case, signal goes through the chopper is the voltage from a single-to-differential amplifier. Here, I plan to use AD8138. There is no illustration of output resistance in AD8138 datasheet. But from the output voltage swing and output current, it should be in the order of several tens ohm. Could I estimate the ouput resistance by this way?

For the integrator, I plan to use LTC 1992 you recommended in another thread as the fully differential amplifier. Its input resistance is 500MΩ. For input resistor I will use, its resistance can be tuned from several hundreds to several thousands ohm. Input current for integrator is 10-100μA.

Do you have some comments for my choices?

Thanks a lot.

Victor

And if I use LTC6943 as one chopper, from my understanding of the datasheet, https://www.analog.com/en/products/ltc6943.html, I need to short the terminals, for example, 9 and 10. But in the datasheet, they are usually connected with a capacitor. So I am not very sure. Could I short them?

Hi Victor,

If you don't mind, I'll study the LTC6943 datasheet myself, it might take a while, okay?

You mean this table?

It is not necessary to determine the output resistance with it, because the short-circuit current and the output voltage in the absence of a load are indicated here. You can refer to this graph

Apparently, the output resistance in differential mode will be equal to twice the value on the graph. By the way, why do you use this driver? Should it be high frequency for your application?

Regards,

Kirill

Hi Kirill,

Of course, I'd like to know your opinions after you studying the LTC6943 datasheet.

I didn't find this graph of output resistance. Thank you for reminding. "Ctrl+F" is a good shortcut that I often forget to use...

The reason why I plan to use AD8138 is that I need to convert an analog voltage to differential voltage. You can refer to the figure below which is my basic idea. I want to drive each capacitor with a random bit stream (about 5MHz). Then the top-plate voltage is converted and amplified to a differential voltage.

Actually I think lower input impedance of single-to-differential amplifier is better considering the settling error (total capacitance of capacitor array is about 2μF). But the single-to-differential amplifiers I find in ADI product list https://www.analog.com/en/products/amplifiers/adc-drivers/single-ended-differential-amplifiers.html are with high impedance.

Does my point of view make sense? What do you think about the parameters I should care about S2D amplifier? I'd like to hear from you.

Thank you so much Kirill.

Victor

Hi Victor,

Unfortunately, I attached the wrong link to the parametric search for switches, note this, there are 25 parts, I have already fixed this. 

Do you want to get pulse differentiation or do you want to transmit the tops of the pulses lossless? If you want differentiation, it makes sense to have low resistance on the input side of the amplifier to get a small time constant. If your goal is the opposite, all reasoning is reversed.

It's not hard to get a low input impedance just by adding an external resistor. Getting high resistance from an unsuitable amplifier is much more difficult.

Regards,

Kirill