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THS4561: Use with ADC3663

Yishan Chen
Intellectual 1890 points

Part Number: THS4561
Other Parts Discussed in Thread: ADC3663, , THS4551, THS4541

Tool/software:

Hi Team,

Customer are trying to use THS4561 with ADC3663, they have some questions about this usage.

Input parameter: Signal Bandwidth 2Mhz, Gain 1V/V

How to set the common mode voltage in their configuration?

Besides, What is the usable frequency range in ADC3663 datasheet which mentioned THS4561:

What is the different between the THS4561 bandwidth such as 60Mhz in 1V/V Gain.

Thank you,

Yishan Chen

  • +1
    TI__Mastermind 24524 points

    Hello Yishan,

    The ADC3663 has a VCM common-mode reference pin which is an ouptut, the typical voltage here is 0.95.  This cannot be set within the ADC3663 and is essentially a 'fixed' VCM value.  This VCM 0.95V signal is routed to the VOCM pin of the FDA driving the ADC.

    The usable frequency range within the ADC3663 datasheet can be understood when considering the FDA specifications.  For the THS4561, the FDA has a 60MHz small-signal bandwidth in a gain of 1V/V.  When you look at the electrical characteristics table of the THS4561, you will also see a gain of 1V/V large-signal bandwidth of 20MHz (Vout = 4Vpk-pk).  The input full-scale range of the ADC3663 is 3.2Vpk-pk; the design guide suggests the amplifier output 3.6Vpk-pk to compensate for glitch filter loss of 1dB.  This is similar to the Vo = 4Vpk-pk on the THS4561 datasheet, meaning we should consider the FDA large-signal bandwidth.  I recommend looking at Figure 7-2 in the THS4561 datasheet.

    Now that we understand the large-signal bandwidth at unity gain (buffer, gain of 1V/V), we can consider how the gain will effect the bandwidth.  Voltage-feedback amplifiers (including FDAs) have a relation between gain and bandwidth known as gain-bandwidth product (sometimes GBW or GBWP).  This relationship shows how the gain and bandwidth are inversely proportional: an increase in gain results in a decrease in bandwidth, and vice versa.  

    If we assume the ADC3663 datasheet is using the FDAs in a gain of 7V/V and assume the FDA has a large-signal bandwidth of 21MHz (or close to it) at 3.6Vpk-pk output swing, we can understand the usable frequency to be 21MHz / (7V/V) = ~3MHz.

    If you consider the other FDAs THS4551 and THS4541, you can observe similar understanding here.  The FDAs also have harmonic distortion measurements and plots in the datasheet; for THS4561 HD2 and HD3 are > -70dBc at 3MHz.  At higher frequencies the harmonic distortion will continue to worsen; when you examine the higher-bandwidth THS4541, the HD2 and HD3 values are < -100dBc at 3MHz (-100dBc and lower is better).

    In summary, you need to consider your ADC full-scale input range (FSR), FDA gain and bandwidth capabilities, supply voltages, common-mode voltages, and distortion requirements to select an appropriate FDA for an ADC driving circuit.

    I hope this helps you proceed with design support.

    If you need, you can reply here or send an email sharing customer information and their circuit if they require design assistance.

    Best,

    Alec