Discrete chopper switch

Hello,

I want to use some single chopper switches in my prototype PCB. However, when I searched in ADI product list, there are only chopper-stabilized amplifiers which usually consist of two chopper switches.

I just want to use one chopper switch in my design which seems like a mixer. So I was wondering if there is a product as I want. For instance, an IC integrates several chopper switches and several clock control signals.

Thanks and look forward to your reply.

Parents
  • 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,

    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

    https://www.analog.com/en/parametricsearch/10624#/p4264=-1e-12|1e-12&p4458=|Bidirectional|I%C2%B2C|Latched|LVTTL|Parallel|Serial|SMBus/I2C|TTL/CMOS%20Compatible&p4683=(2:1)%20x%201|(2:1)%20x%202|(2:1)%20x%203|(2:1)%20x%204|SPST%20x%201|SPST%20x%202|SPST%20x%204|SPST%20x%208&d=sel|0|546|2606|4264|4270|4458|4683|4811|4812|4813|4814

    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

Reply Children
  • 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