Hi, I am attempting to design an analog input circuitry based on the AD7768-4. The application calls for a single-ended, AC-coupled input, with a range of +/- 8V. Max frequency is ~100kHz.
I am looking for suggestions/reference design for this particular application, specifically suggestions as to what front-end amplifier is suitable. I am also open to suggestions to use ADC chips other than the AD7768-4.
The end product is a portable vibration data logger, interfacing with IEPE vibration transducers. Any suggestions greatly appreciated.
The recommended drivers for AD7768 are ADA4945-1, ADA4896-2, ADA4940-1 for Fast Power Mode; ADA4945-1, LTC6363, ADA4805-2 for Low Power Mode; and ADA4945-1, ADA4807-2, ADA4940-1 for Median Power Mode. The AD7768 can operate up to 5V analog supply so you have to design your AFE that will meet the AD7768 input specification. We have an application note for AD7768 AN-1384 that you may found helpful, but in terms of designing the AFE, I would also suggest to ask question to Amplifiers Community so more people from the community can help out. It is necessary to take note some important considerations in designing your signal conditioning to meet your required specifications. If you could also be more specific on the requirement may also help, that is the required offset, gain and noise level etc. for what you want to measure.
The most suitable front-end amplifiers for this type of application are amplifiers like the ADA4610-1, ADA4610-2, or AD8605, AD8606. These amplifiers provide a low noise option which makes them suitable for use in signal chains using low noise ADC's such as the AD7768, AD7768-4, and AD7768-1. The ADA4610 family are particularly suitable for wider bandwidth signals (>50kHz) where THD is important, such as this vibration application.
For an AC-coupled circuit, you can use a relatively large Rin (10k > Rin > 40k), and coupling Cap, with the input stage in an inverting configuration. Use lower Rin if noise is really critical and you can tolerate the lower input impedance.
For the drive stage thatJellenieRhas already discussed below, I would recommend the ADA4945-1*, which can be configured for both driving the ADC and for the single-to-differential conversion. The AD7768/-4/-1 provide a Vcm pin which can be used to provide the common-mode voltage to the ADA4945-1. Depending on the anti-aliasing requirement, you may consider also that this drive stage can be configured as at least a 2nd order multiple feedback filter.
If you want to consider other ADC's then you could consider the AD4003 or some of the other AD4000 series ADC's. The thing to consider if using the AD4000 series is that you may be able to achieve a wider overall signal bandwidth (>110kHz), but there may be a need for higher order anti-alias filter in front of the AD4003 than you would need for the AD7768-1, since the AD7768-1 is an oversampling ADC.
*The ADA4945-1 is currently in pre-release as of February 2019. A released amplifier of similar performance and that is conditionally pin-compatible is the ADA4940-1.