ESP32 ADC preamp inquiry

[freeman]

Hello, I would like to locate the analog specs for the ADC preamp. Seems like I read somewhere that the preamp has 60db of gain (amplification).

Background: My ESP32 implementation will include an external bandpass filter on the analog input (w/ or w/o preamp feature). The low pass filter section has a relatively steep knee (>105db reject at 500Hz above upper frequency limit of the BPF). The high pass section is used to reject frequencies below 200Hz for any low frequency interference (i.e. transformers, motors, power supplies). The project is installed in a shielded enclosure (w/ RF gaskets and feed through caps). An external antenna will be used for the ESP32 wifi. The device will be operated in a high RF environment.

Query:
What's typical and min signal to noise ratio (s/n) of the preamp? I'm looking for approx -80db below reference.

Is preamp gain fixed at 60db or adjustable? If adjustable, the attenuation resolution (i.e. 1 db steps)?

Clarification - Do the input sensitivity ranges available with the ESP32 also include the preamp or just the ADC? If the preamp is not included with this feature, what's its maximum input level (V or dbV). As a final brick wall, I employ over voltage input protection using a fast switching Zener diode to shunt any overvoltage (transient) to ground. If differential input operation is possible, then one for each of the two ADC inputs.

I assume decoupling is required at the preamp/ADC input pins (5-6 and 7-8) to reduce interference (noise) since my BPF will not be close enough to those pins. Is there a typical value range? I think in the 120pF - 470pF range without any pre-signal conditioning like my BPF. I would like to reduce these values when using the BPF, but understand that there will be some induced HF noise from the ESP32 circuitry itself.

[StefanRavn]

I would also be interested in getting some more information about the ADC as well as the programmable gain amplifier.

• What are the available gains in the PGA?
  Which pins can the PGA be used on? As far as i understand, it can not be used on all ADC capable pins.
  What sample rate is it possible to get using the ADC/How long does an ADC conversion take?

[freeman]

Hello StefanRavn. SparkFun has more info about this device on their website (ESP32 Thing product). You are correct that some pins may be used for ADC only, as these do not have pull up resistors as other GPIO pins. On four of the pins they use 270pF caps for stabilization of the ADC performance. The two pins show the Espressif designations, which are similar, but include the letters "p" and "n". I assume that these may mean positive and negative inputs to the ADC (and preamp?), which would indicate operating in differential mode. Therefore the ground (common) would be isolated from the signal path, unlike a single ended input. This would improve signal to noise performance and better cmrr (common mode rejection ratio). There is also a free book from Mr. Kolban called "Kolban's book on ESP32", which is quite detailed. I am currently studying it to see if there's any more info on ADC and preamp operation. If the gain is not programmable, an input attenuator can be constructed from resistors (better than 2% tolerance). For single ended operation it would be a "T" config, for differential, a "H" config. Hope this helps.

[freeman]

Hello... Attenuation has a type adc_atten_t that has four atten settings, 0(ADC_ATTEN_0db),1(ADC_ATTEN_2_5db "2.5db"),2(ADC_ATTEN_6db), and 3(ADC_ATTEN_11db).

[electromania]

Which are these pins exactly on Wroom32 dev kit ?
I noticed it has two pins named P and N.
How to use this preamp or differential amplifier using arduino IDE or eclipse ?

[ESP_Sprite]

The LNA is described in the ESP32 technical reference manual, around page 448. Unfortunately, as far as I'm aware there's no drivers to use it from ESP-IDF/Arduino yet.

[WiFive]

The LNA is described in the ESP32 technical reference manual, around page 448. Unfortunately, as far as I'm aware there's no drivers to use it from ESP-IDF/Arduino yet.

TRM, like for most things, is heavy on high-level capabilities and light on low-level operations. IMO in general it is quite hard to go from TRM to driver code. I don't think the RTC ADC FSM are documented and there is no mention of registers like SENS_SAR_MEAS_WAIT2_REG.

[ESP_Sprite]

With regards to the LNA, I'm afraid I have to agree with you on that. I'll see if I can get the people to put some more information in there; the base assumption of the TRM is that it should contain enough information for people to write their own drivers.

[anichang]

Hi,

is it possible to use the preamp to hook up a microphone?