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AD9674 TGC Gain input driver


For AD9674, we need a differential driver for Gain+-, in datasheet figure38, there is reference design, but this design need an input from -1.6V to 1.6V, except a DAC, we need an OP amplifier to convert to bipolar output. and for this amplifier, need a negative power supply.

So I want to know is, is there a more simplified driver? Googled and found Linear has a design(dn454fa), can we use this design? for this design, Can we use op like AD8138? Is there any DAC recommend for this design? Thanks!

  • Hi Gsing,

    Thanks for selecting AD9674.

    For the TGC gain control circuit with an input of -1.6V to 1.6V, actually you do not need a negative supply for both the differential circuits or single end to differential  converion with the circuits you attached. I'd like to say both of them can work with a single +5V power supply.

    I do not think AD8138 is suitable here for the -1.6V to 1.6V with a 3.2Vpp, as you can find in its datasheet, it is not a rail to rail output amplifer, with a +5V power supply, its output voltage swing is only 2.9Vpp, which can not meet the 3.2Vpp requirement.

    I suggest you to use ADA4940-1, which is a rail to rail output differential amplifier, although these's still 0.1V room required between the power supply and ourput. And it can support both differetial and single to differential circuits to provide -1.6V to +1.6V with a +5V power supply, however you may need a 0.9V Vocm, forturnately it is acceptable for the AD9674 if you look into the datasheet.

    You can download the Diff-Amp Calculator from to simulate and design the circuits by yourself.

    For the DAC, I am assumimg your application is ultrasound, so AD5450/1/2 single channel DAC for a typical single end to differential TGC gain control circuit, and you can use AD5429/39/49 dual channel DAC for a total differential TGC gain control circuit.

    Hugh Yu

  • HI Hugh,

    Thanks for your help!

    I want to use AD9674 at ultrasonic NDT.

    I am newbie in this topic, so I spent some time to learn it. If my understanding is wrong, please correct me, thanks!

    For the first circuit, if using single positive supply, then input signal VinP should be added an offset, also add an offset in VinN, then seems this circuit is similar with Linear's solution.

    For the second circuit, the two inputs should be complementary, but the AD5429 output is not complementary, I even did not see too much difference with AD5450 at analog output.

    I complain about negative supply, because  I need a simplified solution for this Gain, if there is not, then negative supply is also OK. So do you have any verified reference design that can share with us for this Gain driver?  Thanks!

    if using the negative supply, I found a circuit as below, although I still not understand it. If can not find other design, I will consider to use it and ADA4938. 

    By the way, the datasheet of AD5541 as below red marked should be a typo, output should be bipolar, not unipolar.

  • Hi Gsing,

    With the negative power supply, the circuit is easy, and the ADA4938 is the right part. Using the circuits in the CN0181 and ADA4938 is ok for your design, but I do not think the ADA8657 is good candidate due to bigger noise and small GBP. I will recommed you to use ADA4896-2.

    With the single power supply, The AD5429 has 2 channels of current output, you can set different codes to output two complementary output voltages (one is -1.6V to 0 and the other is 0 to -1.6V), and then you can connect these two output to the ADA4940, so you get a true differential circuits. You can use both the AC coupled and DC coupled for the ADA4940 input.  

    AD5450 is a single channel DAC, so it can only support a single end to differential output with the circuit I answered last time.

  • Hi Gsing,

    Sorry for the single power supply, you need add an offset voltage (1.6V) to bring the output voltage up. Otherwise you still need a negative power supply to covert the DAC output current to voltage.

  • Thanks Hugh, sorry for the late respond.

    I prefer below solution:

    cn0180(ad8657 change to ADA4896-2) + ADA4938

    Because I can not understand fully for other choices. It is high risk for me.

    For "The AD5429 has 2 channels of current output, you can set different codes to output two complementary output voltages (one is -1.6V to 0 and the other is 0 to -1.6V)", do you mean use one AD5429 chip to output complementary outputs? but it still need a negative power supply to output -1.6V.

  • Hi Gsing,

    You are correct, it still need a negative power supply to output -1.6V unless an offset voltage (1.6V) is added to bring the output voltage up.