Part Number: ADS1292R
Other Parts Discussed in Thread: OPA320, , LMV321A, LMV321, ADS7042
In the reference design (TIDUB75.pdf), an OPA320 is used. In this E2E thread, Brian Pisani wrote, " I chose an OPA320 since it has a very wide bandwidth and high input impedance." This is a precision, low noise amp as well. The slew rate is 10 V/microsecond. The reference design uses the OPA320 in unity gain configuration.
Those are great features, but the OPA320 has a nominal current of 1.45 mA while the ADS1292R consumes only 335 microwatts per channel. That is, adding the suggested pacer detection conditioning circuit consumes much more power than all the rest of the ECG circuit.
Why was the OPA320 chosen based on very wide width and high input input impedance?
Re: very wide bandwidth (and also slew rate): Since the output of the programmable gain amplifier (PGA) has a 8.5 kHz bandwidth and the maximum amplitude of the pacer pulse testing required IEC standards is 250 mV, and the reference design includes a unity gain design, it is hard to understand why a very high bandwidth OA is required. Considering the slew rate and the narrowest pulse of interest, 100 microseconds, we see that 10 V/microseconds is much higher slew rate than is needed here, the slew rate allows reaching 250 mV in about 0.025 microseconds.
Re: high input impedance. On the 129x ICs, the PGA1P and PGA1N pins are driven by op-amps through an internal series 2k resistor (per figure 22 of ADS1292R.pdf). That is, we don't have a high (10 MΩ) impedance driving the op-amp. The only comment I found relating to impedance is in the reference design (but it is unrelated to the amplifier's input impedance) "However if gain is needed, the series combination of Rs and Rf should be higher than 100 kΩ since the VCAP2 output has limited drive strength."
Please help me understand what I'm missing here. Is there a lower power TI op-amp that will work for the circuit suggested in Software Pacemaker Detection Reference Design (TIDUB75.pdf) ? How about the LMV321A?
