Other Parts Discussed in Thread: TIDA-01471, ADS127L01, THS4551, OPA2320, DAC80501
Dear,
Recently I am working on the IEPE Sensor analog front circuit, and found TIDA-010249 reference design was helpful. (TIDA-01471 reference design was good too).
In the design, I can understand the AC coupling signal paths, including attenuation, program gain, Low-pass-filter (differential drive for ADS127L01), etc. But a little confused about the DC coupling implement.
In TIDA-010249, my understanding is C15D can be short, and VCOM1 should connect to VDAC from external DAC8050, so as to fulfill DC coupling signal paths.
I guess the VDAC was used to shift the DC offset of input signal, since DC components of input signal cannot be filtered by C15D now. Without the shift, the input signal after attenuated can make the amplify of next stage into saturation.
The concern is, how can I determine the value of VDAC? In AC coupling paths, the shift voltage is VCOM (ADD/2); In DC coupling, the dc components of input signal can be different.
For example, if the input signal is 8V(DC) + 5Vpp. After 1/8 attenuation, the signal to next stage will be 1V(DC) + 0.625Vpp. If VCOM (ADD/2) is 2.5V, the VDAC should be 1.5V. In actual situations, I did not know exactly the DC component, so I may set the VDAC 1V or something else. With that, amplify in next stages may go to saturation state? And ADS127L01 will sampled wrong data?
The other question is, in DC coupling signal paths, should the VCOM pin of THS4551 connect to VCOM (ADD/2) , or connect to VDAC of external DAC8050? How about the negative input of OPA2320 (component U5AD)?
Thanks very much for your help,