I have to realize an A/D board with four ADCs (AD7730). The input signals are in 0-10 mV range, and I need at least 14 bits of resolution or greater. I have built an evaluation board to test converters' performance, but I am not satisfied. I have noted that the sclk signal is very critical and the adc introduce a lot of noise (I have only 10 bits of resolution). Nevertheless I think that the problem is my demo-board and not adc part. 1) Could you send me any information about AD7730, layout and routing circuit problems ? 2) How can I reduce adc's noise effects ? 3) How can I deliver sclk signal to four adcs ? 4) Is it usefull amplifing and filtering input signal to reach max adc input range ?
To properly specify the expected resolution of the AD7730, you must need to quote, the input range you are using, output update rate and the status of the CH=P BUF and BIPLOAR bits. You can then refer to the noise tables on pages 10 and 11 of the datasheet to determine the expected noise performance and resolution. For example for a 0-10mV input range with a 50Hz update rate, you can expect a peak to peak resolution in excess of 15bits. However, if you set the update rate to 1200Hz, the peak to peak resolution will only be 13.5bits. I would tend to agree with your assessment that the most likely cause is the layout of your PCB. The device noise of the AD7730 is of the order of 160nV rms provided the bandwidth is kept low by keeping the output update rate low. As you increase the bandwidth, more device noise will be present in the output and eventually as you increase the bandwidth further, the quantisation noise of the conversion process will start to dominate and the resolution will fall off drastically. 1) We have an evaluation board available for the AD7730, details are given on our website. The eval board shows the recommended layout for a single AD7730. A few notes on grounding for sigma delta converters follow: The pin marked DGND on the AD7731 is the return path for the digital current out of the AD7731. It does not necessarily mean that this pin should be connected to the digital ground plane of your system. Connect the AGND pin and the DGND pins of each individual IC together with a broad, low inductance track. You can now treat each of these seven nodes as a single analog ground connection to be routed onto your system analog ground plane. You should provide a separate ground return path back to the power supply for each converter in the system. If each AD7730 has 7mA of analog current and 1mA digital, providing separate return paths means that there is only ever 1mA digital current in each converters local ground plane. You can do this by flooding the analog ground plane and sectioning it off with cuts to provide a local ground return path for each converter. The digital current from the AD7731 is relatively small in comparison to the analog current and provided you use low inductance ground paths this small amount of digital current should not adversely affect the noise floor. You should ensure that AVDD is decoupled to AGND and DVDD is decoupled to DGND. The system analog ground plane should be kept separate from the system digital ground plane, which routes the return current for the “spikey”, digital components such as micro-controller, digital buffers, logic etc. The connection between the system digital ground and system analog ground should be made at a single point (commonly referred to as the “star point”). Where you place the star point will depend on your system but this is often determined by the power supply you use. The single connection between analog and digital ground is already made at the power supply. Your job as a designer is to ensure that no digital current is allowed to flow in the analog ground plane while ensuring that the potential of the analog and digital grounds in your system remain equal. The final chapter in all our seminar books is dedicated to hardware design techniques and deals with such issues as grounding, decoupling, parasitic thermocouples and good PCB design. http://www.analog.com/support/standard_linear/seminar_material/index.html 2) The ADC is not inherently noisy. As described above the noise in the ADC is a trade off between update rate and resolution required. If you have noise in your system which is limiting the peak to peak resolution to 10bits, this could be due noise or spurious signals on the inputs, a noisy power supply ( if you have a switched DC DC converter in your power supply, this can generate a lot of noise), or it could be due to poor grounding in the system which allows high frequency digital currents to move the ground potential around. 3) The SCLK is not related to the master clock frequency in any way. You should have a separate crystal supplying the master clock to each ADC. Your system should then read each ADC in response to DRDY going low. You can only address one ADC at a time therefore the delivery of SCLK to all four ADCs should not be critical. Where you might have an issue is if SCLK is not shielded and can couple into the input or the power supply (or both). When tracking SCLK around your system, ensure that you have ground below at all times and ground on both sides of the SCLK track. Look for places in your PCB where SCLK could couple onto other lines. 4) Amplifying an input signal which starts in the 10mV range will almost certainly add more noise into the signal of interest. Most opamps will be dominated by 1/f noise, and generally in sensors it's the low frequency information you are interested in. Designing a pre-amp will be a very difficult design job. I would suggest that interfacing directly to the AD7730 with a single RC anti-alias filter will yield the best results in the shortest time. Once you can get the AD7730 operating correctly you should be able to achieve up to 16 bit peak to peak resolution.