Shunt voltage reference buffering - Page 1
Forum section: electronics / projects

author: posatomic
author_rank: Contributor
timestamp: February 06, 2022, 06:27:16 pm
content: Hi!I have a question to the more experienced.I am developing a data acquisition board as the current part of my project, and I have little to no experience with high speed pipeline ADCs and supplying a proper reference for them.When I designed the schematics, I thought adding a buffer to the shunt voltage reference might be a good idea to begin with, see attached image.My question is if this even achieves anything, or if it does not, does it negatively affect the reference's performance? Do people buffer the output of shunt references or are they just connected to the ADC directly?

author: TimFox
author_rank: Super Contributor
timestamp: February 06, 2022, 07:03:42 pm
content: Look at the specified output impedance of the voltage reference device in its data sheet, and the input impedance of the ADC in its data sheet.If the output impedance of the reference is already low, the buffer will only add noise and drift to the voltage fluctuations at the ADC input.The AD data sheet shows atypicaloutput impedance for the buffer at low frequencies of only 0.2 ohms, but it rises above 500 Hz to 400 ohms at 1 MHz.The reference data sheet gives amaximumoutput impedance of 0.3 ohms (no frequency dependence published).If the input current to the ADC pulsates, the capacitor may be needed at the ADC input to hold charge. Otherwise, connected directly across the voltage reference will have little effect, since the impedance of the reference is low.If you need to filter the current from the power supply into the reference, splitting the resistor in two series resistors with a capacitor from the midpoint to ground will have a stronger filtering effect on power supply ripple.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 06, 2022, 07:10:03 pm
content: Whether you need buffering for a shunt reference will depend of the ADC input, the current it draws. The ADC may already have a buffer or not. You could also use a series referenceAnyway the AD8244 seems to have too high DC offset, look for something with lower offset and low temp drift, and low noise, OPA187 for example

author: posatomic
author_rank: Contributor
timestamp: February 06, 2022, 08:15:22 pm
content: https://www.analog.com/media/en/technical-documentation/data-sheets/ad9215.pdfAccording to the datasheet for the ADC, the reference pin has a 7k input resistance. At 1V, this is about a 142 uA current load.Quote from: MasterTech on February 06, 2022, 07:10:03 pmYou seem to be absolutely correct, thank you for that insight! The buffer basically doubles the temperature drift of the reference if I am correct? This makes me tempted to just remove the buffers.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 06, 2022, 09:01:03 pm
content: It looks like the 7 K input impedance is an actual resistor and there seems to be an internal buffer / amplifier for the refrence. So there is likely no need for a an extra buffer. It still depends on the reference.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 07, 2022, 05:07:59 am
content: The datasheet says: "When the SENSE pin is tied to AVDD, the internal reference is disabled, allowing the use of an external reference. An internal reference buffer loads the external reference with an equivalent 7 kΩ load. "So, it has some internal buffer.