R2R DAC  OPAMP - Inverting or Non-Inverting - Page 1
Forum section: electronics / beginners

author: fc3000
author_rank: Regular Contributor
timestamp: June 16, 2023, 07:57:09 am
content: Hi all.I want to build a 2R2 DAC using discrete components.I have noticed that all the examples online, show the OPAMP driver as inverting.Why is it so?So I used LTSPICE and made a 12bit 2R2 DAC with a voltage follower OPAMP.It works fine!.What are the merits of using an inverting OPAMP? What am I missing?I would appreciate any clarification on the matter.Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: June 16, 2023, 08:34:50 am
content: The interted R2R configuration is attractive because the switches are all working at an essentially fixed voltage near zero and see only low voltage. This makes it easier to get good R_on matching and gate control. This is especially the case for a mulitplying DAC, where the ref. voltage can vary over a large range (in some implementations even negative). The downside is that one has the feedback resistor as an additional factor to possibly add gain drift.

author: fc3000
author_rank: Regular Contributor
timestamp: June 16, 2023, 10:28:15 am
content: Hi.Thank you for the reply.But I still not get, why designs show the OPAMP driver as inverting one.Is it better than a standard unity-gain OPAMP?What are the merits of these designs?Regards

author: magic
author_rank: Super Contributor
timestamp: June 16, 2023, 10:32:53 am
content: The inverting (transimpedance) amp shown in most audio DAC datasheets is necessary for "inverted" or "current output" R2R DACs, or for other current output DACs.Unity gain buffer is necessary for "voltage output" R2R DACs, or other voltage output DACs. If this works for you, it means you have probably built the voltage output DAC and everything is OK.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 16, 2023, 10:40:34 am
content: The inverting configuration usually has the output of the actual DAC part to a virtual ground. So the OP-amp is not so much an inverting voltage amplifier but more lika a current to voltage stage.

author: fc3000
author_rank: Regular Contributor
timestamp: June 16, 2023, 10:59:17 am
content: Hi Magic. Thank you for the replyIf i understand correctly, in a non-inverting buffer design, I could drive a MOSFET directly because now I have a voltage driver?Regards

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 16, 2023, 11:03:11 am
content: Also note, that to get full resolution out of an12 bit R-2R DAC (not 2R2) the resistor for the most significant bit needs an accuracy of about 1/2^12), which is around 0.025%. Resistors up to a tolerance of 0.1% are quite affordable (12ct or so) but if you want to buy more accuracy then it get's quite expensive quickly.For DIY, it's quite doable (and highly advised) to sort your resistors. Use the most accurate resistors for the most significant bits, and use the less accurate for the lower bits, because tolerance is not important at that end.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 16, 2023, 12:09:05 pm
content: It is not just the resistors that need to be accurate, but the combination of resistor and the switch.In the inverting / current ouput configuration the switches are all at the same voltage. With the non inverting / voltage output form half of the switches are at ground and half are at the reference voltage. This makes the matching of the switch resistance more difficult. This is of cause unless you want an extra buffer amplifier for each bit.

author: fc3000
author_rank: Regular Contributor
timestamp: June 16, 2023, 12:44:32 pm
content: QuoteI never considered this! That show how ignorant I am.This is great forum. I learned something new today.Thank you to all of you for the great contribution.Regards

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 16, 2023, 01:39:16 pm
content: 12bit is already quite accurate, but not extreme yet, If you want "more", then small details become more important quickly.You have not stated any end goal for your R-2R DAC. Do you need speed?If it's (mostly) about DC, then PWM is a valid option. You need a lot less critical parts, but the tradeoff is more filtering and low frequency response. I never finished the project below, but the thread is worth reading for a lot of related details.https://www.eevblog.com/forum/projects/making-a-pwm-based-voltage-standard/msg3663823/#msg3663823