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My ADC (LTC2323, 16 bit, 5MSps, http://www.linear.com/product/LTC2323-16) has an internal reference (4.096V) which is provided at output pins REFINTx. Now I need (roughly) half of this reference voltage for the ADC driver. The reference output is buffered and referred to another pin REFRTNx - to my understanding for proper decoupling but otherwise "close enough" to GND. The datasheet does not explicitely confirm or deny that I can load this pin externally but it somewhat suggests I can. What I found in the datasheet:

REFOUT1 (Pin 12): Reference Buffer 1 Output. An onboard buffer nominally outputs 4.096V to this pin. This pin is referred to REFRTN1 and should be decoupled closely to the pin (no vias) with a 0.1µF (X7R, 0402 size) capacitor and a 10μF (X5R, 0805 size) ceramic capacitor in parallel. [...]

REFRTN1 (Pin 11): Reference Buffer 1 Output Return. Bypass REFRTN1 to REFOUT1. Do not tie the REFRTN1 pin to the ground plane.

Under "Internal Reference Characteristics" it is stated: REFOUT1,2 Output Impedance: 0.25 Ohm.

Furthermore:

Internal Reference: The LTC2323-16 has an on-chip, low noise, low drift (20ppm/°C max), temperature compensated bandgap reference. It is internally buffered and is available at REFOUT1,2 (Pins 12, 26). The reference buffer gains the internal reference voltage to 4.096V for supply voltages VDD = 5V and to 2.048V for VDD = 3.3V. Bypass REFOUT1,2 to REFRTN1,2 with the parallel combination of a 0.1µF (X7R, 0402 size) capacitor and a 10μF (X5R, 0805 size) ceramic capacitor to compensate the reference buffer and minimize noise. The 0.1µF capacitor should be as close as possible to the LTC2323-14 package to minimize wiring inductance. Tie the REFINT pin to V DD to enable the internal reference buffer.

Can I load this pin externally and if yes - how much? Can I attach a resistive divider of, say 10k, to get 1/2 of the ref voltage? Or at least the input of a simple op-amp follower?

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Yes, you should be able to load the reference output. I wouldn't load it with much less than \$500\Omega\$, but you should be fine with something above that.

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  • \$\begingroup\$ That sounds great! Would you mind adding how you arrived at this conclusion? Either experience or calculations/datasheet and if so - how? \$\endgroup\$ – divB Feb 6 '18 at 0:56
  • \$\begingroup\$ The data sheet lists the maximum pin current at 10mA, at 4.096V a 500Ohm load draws 8mA. \$\endgroup\$ – C_Elegans Feb 6 '18 at 1:09
  • \$\begingroup\$ Are you referring to the "Isink" symbol in the datasheet (on page 4)? This is for the digital I/O pins. I did unfortunately not find more specs on the reference pin other than 0.25 Ohm output impedance (page 3 bottom) \$\endgroup\$ – divB Feb 6 '18 at 3:36
  • \$\begingroup\$ Yes, that's what I was referring to, as that's the best figure they've got. \$\endgroup\$ – C_Elegans Feb 6 '18 at 3:39

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