Post Go back to editing

About the SFDR and THD specification of AD3551R and LTC1668

Category: Hardware
Product Number: AD3551, LTC1668


We find a new DAC of ADI, AD3551R. This might be the right product for our bio-instruments. We would like to generate a tone with high precision (THD>90dB) within the 1KHz to 1.2MHz frequency range, with high precision, high speed and low temperature drift.

1, Figure 33 and Figure 36 only show the THD v.s. frequency within the range of 100KHz, does AD3551R supports generating a tone above 100KHz, i.e., 1MHz tone? What's the specification of SFDR and THD of AD3551R (as well as LTC1668) within the frequency range 1KHz ~ 1MHz? What's the recommended high speed, precision amplifier suggested to be used with AD3551 in the high-frequency end (800KHz ~ 1.2MHz)?

2, We found that LTC1668, page 5, Figure 6, The SFDR v.s. frequency. LTC1668 could generate a single tone of 1MHz with >90dBFS SFDR with a update rate 5MSPS. Is AD3551R superior or inferior to LTC1668 in generating a tone of high frequency, 800KHz ~ 1.2MHz? 

3, which one should I choose for our product, AD3551R or LTC1668?



  • Hi Benuyan,

    We have not concluded the evaluation of the AD3551R above 100 kHz. The AD3551R is capable of generating tones up to half the update rate but overall performance depends on the amplifier, filter and configuration of the DAC.

    From the amplifiers we have tested, the AD8065 would be close to meeting the 90 dBc THD requirement at 1 MHz. We are still testing other options aiming at improving DC and AC performance.

    The main difference in performance between AD3551R and LTC1668 is the noise floor: 20 pA/√Hz vs 50 pA/√Hz. Our preliminary measurements show that the AD3551R would be on par with the LTC1668 in terms of THD at 1 MHz. However there are several adjustments we are testing to improve performance. We aim at publishing an application note with recommendations when the work is concluded.

    Best regards,


  • Hi Antonio,

    Thank you! We are unsure about the performance of AD3551R within the frequency range of 800K ~ 1.2MHz, as these frequency bands are vital to our bio-impedance applications. So it is safe to design using AD3551R right now or we should wait for your application note?

    Apart from the noise floor, we also found that AD3551R may have better temperature stabilities. This chip as well as its two-channel variant are very attractive for our applications. Could we order some samples or 10 pcs right now via a distributor in P. R. China?



  • Hi Benuyan,

    If your project has a stringent timeline, I suggest you proceed in parallel with our evaluation since it will take several months to complete. You may need to build your own circuit since the evaluation board may not achieve the required performance.

    The purpose of EngineerZone is to provide communities with the support of select Analog Devices products. For additional information on pricing or product availability, we recommend contacting your FAE or local ADI salesperson.

    Contact details for your local ADI salesperson are available via the following page on the Analog Devices website: Sales and Distribution | Analog Devices

    Best regards,


  • Hi, Will the reference design of LTC2757/LTC2758 using composite amplifier suit for 1MHz, >90dB THD design for this AD3551R chip?

  • I don't think so. The amplifiers in that design are way too slow, so the output would just oscillate trying to follow the DAC. The topology is probably not adequate since Iout has to dump current into a virtual ground at 2.5V.

    I would recommend a two-stage composite amplifier with ADA4807 as first stage and LT1363 as second stage.

  • Thank you for your support! I compare this two chips, ADA4807 and LT1363,,asc&p4502=50M|4G&d=5047|4501|4089|4095|2687|4502|300|4101|4102|4124|4108|2839|2840|s3|s16|s5&p2687=1&sel=row144|row219

    ADA4807 is superior to all specifications of LT1363 except for the Vs Span MAX. Could we use ADA4807 alone?

    The output of AD3551R is used to drive a Howland VCCS current source injecting ~1mA current in to biological tissues. So the output of AD3551R should be precision, and the span of voltage output can be left to the enhanced howland current source.

  • How about LTC6226/LTC6227?

    I am a little bit puzzled on the selection of DAC output driver buffers for AD3551R, may be I should stick with your upcoming reference designs which features a low distortion solution. Or, could you tell me some design considerations in selecting the right buffers for AD35531R?

    The frequency range is 1KHz~1MHz, so maybe some high-end audio amplifiers could do this?

  • Yes, you may use ADA4807 or ADA4805 if the voltage span is less than 10V. Note that current sourcing / sinking capability is exhausted as the output voltage gets close to the rails. It should be no problem if the Howland amplifier has high input impedance.

  • The LTC6226 is a good candidate as well; it has enough slew rate but perhaps too much bandwidth. The only limitation is that it cannot support the +/- 10V range.

    GBWP and Slew Rate are relevant if you plan on using the DAC to generate fast settling pulses. Working with harmonic signals you should focus on closed-loop gain and THD. You just need enough bandwidth to play the highest tone with suitable amplitude and phase deviation.

    With 20 pA/√Hz DAC current noise, you get 30 nV/√Hz with RFB1 and 60 nV/√Hz with RFB2, so don't worry too much about the amplifier intrinsic noise.