Using a Video ADC as Regular ADC? (TDA8754) - Page 1
Forum section: electronics / projects

author: m.m.m
author_rank: Regular Contributor
timestamp: February 19, 2021, 11:00:30 am
content: In a project, I need to have a >80Msps ADC (with parallel interface, of course).So to reduce the costs I'm planning to use a video ADC, particularly aTDA8754which I found in a local store for ~0.5USD.This IC has triple 8-bit ADC, with a sample rate of up to 270 Msps.I was just wondering if it's possible to use this IC as a regular ADC since it has a few other pins such as Vsync, Hsync & COAST and I do not have any idea about what these pins functionalities are, and if it's not, is there a way to generate the Vsync & Hsync signals with a simple external circuit.Any ideas are welcome.

author: m.m.m
author_rank: Regular Contributor
timestamp: May 01, 2021, 07:26:02 am
content: Does anyone have any experience with this IC?I also found a demoboard application note (AN/10295_1) for that, but still, I cannot get any idea whether what I'm planning to do is feasible or not.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 02, 2021, 10:36:16 pm
content: A long, long time ago a video ADC was used in some scope kit from Velleman. I think it had a TDA8703.You link to the datasheet, but you can also read it.I won't, I despise those datasheet collection sites that force you to click through shitloads of useless circular links hunting for a download button.

author: rsjsouza
author_rank: Super Contributor
timestamp: May 02, 2021, 10:39:19 pm
content: I don't know this particular ADC, but many years back at the university we used some fast Video ADCs for general purpose apllications - they worked well.

author: m.m.m
author_rank: Regular Contributor
timestamp: May 03, 2021, 08:36:06 am
content: Quote from: rsjsouza on May 02, 2021, 10:39:19 pmQuote from: Doctorandus_P on May 02, 2021, 10:36:16 pmI just did a search and found a few projects that use this IC but this IC doesn't actually have the video capabilities. Here's a block diagram from the datasheet:But the TDA8754 which I mentioned has a few other blocks as shown below:So as you can see the analog signal first passes through a CLAMP/AGC (which is not Automatic gain control btw, it's just a variable gain amplifier.) block before getting into the ADCs, and also there's a lot of sync blocks controlling the PLL clock generator. The clamp levels, gain value & sync settings are controllable through the I2C bus.As far as I understood, this IC is intended to receive some AC coupled analog input, and clamp it to the desired DC value.Here's a note from the datasheet:QuoteSo since I do not have any prior experience in analog video, my first question is whether I can DC couple the input and disable the clamping block since I do not want to change the DC level of my signal when I'm converting it. I also need to know if the signal is AC coupled internally again once more because that would be a problem.The datasheet also states thatQuoteDoes this mean that if I just connect the CLP pin to the ground the clamping block would be disabled? I didsome researchand I'm guessing that this IC uses Keyed clamping and if that's the case there could be some hopeAnd my second question is that I just totally don't understand the behavior of the sync blocks and whether they can be disabled.I have also added the datasheet in the attachments.Also, the reason that I cannot use the TDA8703 is that I couldn't find one in my area. I would have to order some for double their price and wait for 2 months to arrive.

author: fcb
author_rank: Super Contributor
timestamp: May 03, 2021, 09:16:41 am
content: TDA8754 looks like it could be bodged to do what you want.The PLL has enough different operational/test modes and options that it should be possible to get a stable (non-coasting) sample clock. Helped by the fact you can output the PLL to a pin to check.Register 0x1B, 0x14 usefulDisabling the clamping looks possible with register 0x15, although don't count on it until you've tested it - there could be some weirdness if PLL coasting is disabled.The ADC itself will be a DC device, just that video is normally coupled into these as AC, then clamped on the back porch (the bit after the sync pulse).

author: m.m.m
author_rank: Regular Contributor
timestamp: May 03, 2021, 10:21:34 am
content: Quote from: fcb on May 03, 2021, 09:16:41 amThank you for the answer, just out of curiosity, what is PLL coasting? I couldn't find that anywhere.

author: fcb
author_rank: Super Contributor
timestamp: May 03, 2021, 01:43:13 pm
content: PLL's require a stable reference clock, if this is derived from video sync's you'll get a disturbance each new frame (due to the way frame syncs work). If this disturbance is fed to the PLL, you'll get a change in the average PLL frequency and (depending on PLL filter) a noticeable shift just after the frame sync.Video PLL's have an option to put the loop filter into a 'coast mode' around the time of the sync pulse to avoid this.Perhaps a way to imagine it: Using a PWM from a microntroller into an RC filter to create an analog voltage, and then momentarily make your PWM pin hi-Z (tristate the pin), the analog voltage would remain the same for some period of time (subject to leakage). This is essentially what the PLL filter does.You won't need coast the PLL in your application, you'll just have to make sure you disable it.

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 03, 2021, 04:37:16 pm
content: If you will have any test results please share. I'm interested in it.The only minor issue I see is they have no specified zero level voltage when used in DC mode, but this can be simply measured (will it be the same level for different batches?)And by the datasheet and their setting time it feels like it doesn't have full bandwidth or have some form of low pass filter built in.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 03, 2021, 05:20:48 pm
content: Back then IC's were much less integrated then these days.Looking up the details in the datasheet to disable all the unused features of this ADC is an exercise that would take too much time for me.Is this for a one-off hobby project, or is there any chance to make kits out of it?The datasheet I saw was 23 years old. and availability can become a big issue quick.If it's for hobby and you can get them for 50ct then just buy a bag of them so you have plenty.If it's for a kit, then be very careful about availability.

author: m.m.m
author_rank: Regular Contributor
timestamp: May 04, 2021, 06:26:15 am
content: Quote from: Doctorandus_P on May 03, 2021, 05:20:48 pmI'm trying to use this IC in my project for the BSc final year, which is making a digital oscilloscope. But I also have a backup plan in case I couldn't make this work, which is to use the internal triple interleaved ADCs of the stm32f4 series which could achieve up to 7.2Msps. There are already a few existing projects using the internal ADC so I was trying to make something new.But the reason I wanted to use this IC is that I'm planning to make it an open-source DIY project and keep the price as low as possible.Also since there aren't many ICs available in my area this could be a good option for me and other people like me.I already bought 4, and I contacted the seller and asked if they could provide them in larger quantities & they didn't give me any specific number about how many they have available but replied that they don't have any limitations on the amount at this moment.I will certainly update this topic if I could make any progress. But until then, if you had any ideas feel free to share them.