Other Parts Discussed in Thread: THS4521, , THS4561, TINA-TI
Hello forum,
We're currently working on an environmental data logging application. We're building the analog front end using the THS4524 FDA and a ADS1274 ADC, which seems to be a common pairing.
However we've hit a few blockers that are halting our progress, and would appreciate any assistance from any experts in the field of analog, specifically FDA implementation.
The input signal
Output topology: Single ended or differential (Manufacture selectable, we want to use single ended)
Output impedance: 50Ohm (single ended), 100Ohm (on each line in differential)
Output bias: Either DC coupled (to Vsupply/2) or AC coupled (Manufacture selectable, we want to use AC coupled)
Output voltage: 0V to Vsupply. (in our case 0-5V)
Signal frequency: 2 - 20kHz.
The ADC
Accepts a differential input voltage between 0 and 5V, DC coupled to 2.5V.
Q1) Differential or single ended?
Since we have the option to select differential output from our source, is there any advantage to using the THS4521 in differential mode? I have read that this simplifies the process of selecting termination resistors, is that true, or can we achieve the same performance with single ended (albeit with a slightly more involved termination resistor calculation?).
Q2) Can we remove the capacitors between the input and output of the THS4521?
We purchased the ADS1274EVM-PDK, which has THS4521 circuits on-board. On the board, it uses 1.5nF capacitors in parallel with 1KOhm feedback resistors on the THS4521 (see image below).
These capacitors form a low pass filter with a cutoff of 103kHz on the output which is undesirable for us.I removed them manually from the board. However this resulted in the signal shown below.
Yellow = input, blue = output.
The input was single ended from a 600Ohm source. When the capacitor is present, the FDA works fine, when it is not present, we see the signal in blue above. Is there a reason for this behavior, are these capacitors necessary for stable function?
Q3) Reliability of the 8802.FDA Design tool for calculating gain, feedback and termination resistances.
I have been using the 8802.FDA Design tool to assist in selecting values for RT, RG, RF and RM based on the single ended circuit below.
When I run this calculator with RF, RG and RIT from the datasheet (page 24, table 5, shown below) for a gain of 1 V/V, the calculator shows a full-swing ADC voltage of 0.25V for a 0.5V input (suggesting a gain of 0.5), am I missing something?
Calculator result
3) Using three FDAs in parallel on the same input signal
We intend to use the THS4524 to apply gains of 1x, 8x and 64x to our input signal. We will placing all three FDA inputs in parallel. Will this cause issues with termination, and if so is it best we use a buffer or isolation transformer on each input?
Apologies for the length of this post, but it will be an incredible exciting application for TI's products if we can get this to work.
Thanks in advance,
Josh