I am using the AD5687BCPZ DAC to drive an RF circuit. The control is from an FPGA. I have two different grounds, i.e. an RF ground for the RF circuit and a digital ground for the FPGA. In some DACs, the analog and digital grounds are clearly specified on the chip. Can you help advise if I should place the AD5687 DAC on the RF ground or digital ground or should I separate the grounds going to the DAC?
The analog and digital ground connections is always a controversial question so, let me explain/argument my opinion,
Internal digital and analog grounds are some times bond separately to improve energy dissipation... as you can imagine, at this levels of miniaturization the parasitic are not negligible so, physically, cannot achieve a zero ohms connection, and any internal energy transition will affect the other block (analog - digital). In conclusion, to minimize this disturbance, ie digital crosstalk, separate bonding is used.
From a practical point of view, both grounds must be connected as close as possible to minimize current loops... in other words, analog can digital must be connected together at the output of the device... at this point, due to higher geometries used at the output, the impedance are smaller, so, disturbance can be minimized pretty good.
In your case, connect both GNDs to AGND.
Thank you for the reply. It is very insightful.
This chip has 3 bias lines, namely Vref, Vlogic and VDD. I will bias the Vref from a precision voltage reference. Would it be possible to bias the Vlogic and VDD from the same supply? My concern is that the chip is on the analog ground and if I bias Vlogic from a digital supply, the return currents may be large compared to biasing both Vlogic and VDD from an analog supply.
Sorry about the delay.. I didn't get any email alert about your answer.
Yes, you can do BUT remember to add independent decoupling caps on each supply pin.