Balanced line level input for PCM1804 without negative supply - Page 1
Forum section: electronics / projects

author: Zipdox
author_rank: Regular Contributor
timestamp: September 14, 2024, 09:03:56 pm
content: I want to add a balanced line input to my device, and I chose the PCM1804 as the ADC. Line levels fit inside the input range of the PCM1804 so I don't think need any amplification. I don't want to have to create a negative voltage supply rail. The PCM1804 has a common mode voltage output (2.5V) on the Vcom pins. Is it okay to just capacitively couple the input with 10k resistors to the common mode voltage, combined with some ESD protection diodes like this? I read on TI e2e that the Vcom pins can only output a few hundred uA, is this a problem? Should I buffer Vcom? If so, what opamp should I use for that?

author: moffy
author_rank: Super Contributor
timestamp: September 14, 2024, 11:52:36 pm
content: Unfortunately that won't work for this CODEC particularly the input ADCs. Looking at the datasheet:https://www.ti.com/lit/ds/symlink/pcm1804.pdf?ts=1726357194655&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FPCM1804'figure 44' The differential inputs need to be driven by a low impedance source, hence the use of the 10nF cap, the 47R resistors and the opamps. The input to the ADC produces dynamic sampling glitches that need to be controlled by a low input impedance source, which also needs to be close to the ADC for best effect.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 15, 2024, 12:52:34 am
content: Yes, you'll have to add a preamp in between. A true instrumentation amp/or "microphone" preamp (which is basically just an instrumentation amp) You'll use the VCOML rail to bias its reference.

author: Zipdox
author_rank: Regular Contributor
timestamp: September 15, 2024, 02:27:49 am
content: Quote from: SiliconWizard on September 15, 2024, 12:52:34 amDo you have any recommendations for an instrumentation amplifier?

author: Zipdox
author_rank: Regular Contributor
timestamp: September 15, 2024, 03:02:27 am
content: Okay I've found some fully differential audio op-amps from TI (OPA1632 and OPA1633) that show the PCM1804 as an example in the datasheet. What a coincidence.

author: Zipdox
author_rank: Regular Contributor
timestamp: September 16, 2024, 01:47:14 am
content: The OPA1632 datasheet recommends 390 ohm for Rg and Rf. Wouldn't this lead to a low input impedance? Should I buffer the input? If I'm already buffering the input, can't I just send that directly into the ADC?

author: Zipdox
author_rank: Regular Contributor
timestamp: September 21, 2024, 04:20:00 pm
content: Could I not just use OPAx376 to buffer the input directly like this?

author: magic
author_rank: Super Contributor
timestamp: September 21, 2024, 05:16:39 pm
content: This should work if the opamps are R2R.Beware that many RRI opamps have distortion in the region (typically near VCC) where they transition from NPN/N-ch input stage to PNP/P-ch. TI has some CMOS opamps which internally generate a higher supply rail, allowing them to work exclusively with P-ch input devices and avoiding this problem, but I forgot exact part numbers. Another solution is to use a higher supply voltage, if available.

author: MasterT
author_rank: Frequent Contributor
timestamp: September 21, 2024, 07:05:46 pm
content: OPA with "zerocross" feature from TI are opa2365, opa2320 and -22 -25.I don't think opa4376 is a good choise, slew rate is low. ADC is required RC network at the input, to lower current spickes that disstress opa, otherways > 1 GHz GBW 'd be necessary. The problem none of the product capable to satisfy both condition - low THD and extremely high GBW same time.TLV2365 low cost alternative for OPA series, my recomandation, ( with RC network).If slightly higher THD is not an isue, than very cheap THS7316, THS7373. Tests show that series have very good drive capability, so RC-kickback 'd accomodate higher C value (0.1uF for audio) and lower R, 10 OHms or so. Of cours THS is better for high freq. application.

author: Zipdox
author_rank: Regular Contributor
timestamp: September 22, 2024, 01:59:00 am
content: Quote from: MasterT on September 21, 2024, 07:05:46 pmTHS7316 and THS7373 are video amplifiers, very peculiar and not stocked with many suppliers. Is rail-to-rail really necessary? If the input approaches the supply rails then I think the signal will already be clipping, and it shouldn't reach this with line level signals. So is buffering the input lines directly a good topology? What are some criteria I should like out for when picking an op-amp for this application, and what circuit should I use (filtering and such)?

author: MasterT
author_rank: Frequent Contributor
timestamp: September 22, 2024, 04:40:49 am
content: >>> Is rail-to-rail really necessary?Nice to have, R2R otput of the OPA greatly simplify interfacing, because when Vcc of the ADC and Vcc of the OPA is the same you don't care about adc input protection against higher voltage presents. They used 47 Ohms (PCM1804 data sheet) for double purposes, as filter & OCP / OVP. Resistor at adc inputs creates non-linearity and THD , so value can't be too high, same time with +15V input injected current may reach ~200 (!) mA or whatever OPA can source.>>> If the input approaches the supply rails then I think the signal will already be clipping, and it shouldn't reach this with line level signals.Depends on opa, this is why "zerocross" was introduced. Get DS of tlv2365 and check input offset voltage vs Common Mode voltage.THS73xx series has gain x2, so input CM is not an issue.>>> So is buffering the input lines directly a good topology? What are some criteria I should like out for when picking an op-amp for this application, and what circuit should I use (filtering and such)?If 10k impedance at the entry point is ok than inverting topology simplify OPA selection. For high inpedance (100k-10Mohm) non-inverting topology of course. Using tlv2365 gain x1 (follower) is possible, for others OPA gain >2 required to limit inputs voltage.

author: magic
author_rank: Super Contributor
timestamp: September 22, 2024, 07:23:10 am
content: Yes, inverting topology with RRO opamps running at 5V makes sense here. And it would only need two opamps, not three.Resistor noise is a concern, but here they contribute 10nV/rtHz per side, or 14nV/rtHz differentially, or 2μV RMS over audio BW. PCM1804 full scale input is 3.5V RMS, so over 120dB above resistor noise. This may work out OK, I think.

author: Zipdox
author_rank: Regular Contributor
timestamp: September 24, 2024, 01:34:44 pm
content: Quote from: magic on September 22, 2024, 07:23:10 amWould using an OPA1632 with 10k resistors in the below configuration be a good solution or is using two RRO op-amps in an inverting topology better?

author: magic
author_rank: Super Contributor
timestamp: September 24, 2024, 06:36:08 pm
content: I haven't used this part and the datasheet is not very clear, but the 1632 appears to have somewhat limited output voltage swing, so it probably won't make full use of the ADC's input range (unless you give it wider supplies, like -5V to +10V). Using a dual RRO opamp in inverting configuration "just works".Another thing is that there are internal resistors on the Vocm input, dunno if the ADC would like them (maybe it wouldn't care, but it's something to check).