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Constructing an automatic RCA video camera feed splitte

 
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alec(at)alecmyers.com
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PostPosted: Thu Oct 19, 2017 4:47 am    Post subject: Constructing an automatic RCA video camera feed splitte Reply with quote

I think we’re ok. That section is either wrong, or badly written, I can’t work out what it’s trying to say either. But:

Look at table 1 in para 8.4 - control L gives High-Z at the output.
9.2.1 says “Avoid bus contention because it can drive currents in excessof maximum limits.” - it’s clearly anticipated as a bus driver IC.
On the left of figure 16 shows output pins 2,3,9 and 10 tied together to form the output of the 4-channel mux in the sample application circuit.

AD8184 also needs a split supply. I don’t think you’ll find any CMOS devices that will switch an analog signal outside the supply rail, it’s not compatible with the tech.


Quote:
On Oct 19, 2017, at 2:04 AM, Eric Page <edpav8r(at)yahoo.com> wrote:



Gentlemen, we have a problem. Unless I'm very much mistaken, the CD4066 is not going to work. I'll be quite happy if someone will tell me I'm wrong, but...

In the CD4066BE datasheet (http://www.ti.com/lit/ds/symlink/cd4066b.pdf), at paragraph 8.1 it says,

CD4066B has four independent digitally controlled analog switches with a bias voltage of VSS to allow for different voltage levels to be used for low output. Both the p and the n devices in a given switch are biased on or off simultaneously by the control signal. As shown in Figure 17, the well of the n-channel device on each switch is tied to either the input (when the switch is on) or to VSS (when the switch is off). Thus when the control of the device is low, the output of the switch goes to VSS while when the control is high the output of the device goes to VDD.

I read this to mean that we can't tie eight outputs (from two ICs) together into a common VIDEO OUT bus because one "on" channel will be driving a video signal into seven others that are sitting at -5V. This IC is four independent analog switches; tying their outputs together looks like a recipe for silicon smoke.

It gets worse. At paragraph 8.2 (3) it says,

Normal operation control-line biasing: switch on (logic 1), VC = VDD; switch off (logic 0), VC = VSS. [VC is control voltage.]

So, the switches in this device don't turn fully off at 0V; they must be driven to Vss (-5V). Obviously, the PIC can't do that. At paragraph 9.2 it presents a typical application very similar to this effort, and suggests using the CD4054B CMOS LCD Driver to level-shift 0 to +5V digital signals into -5 to +5V signals. That's two more 14-DIP ICs, but it doesn't solve the first problem.

I think we're stuck with the AD8184, as it's a proper 4-to-1 multiplexer.

Thoughts?

Eric




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Eric Page



Joined: 15 Feb 2017
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PostPosted: Fri Oct 20, 2017 7:53 am    Post subject: Re: Constructing an automatic RCA video camera feed splitte Reply with quote

alec(at)alecmyers.com wrote:
I think we’re ok. That section is either wrong, or badly written, I can’t work out what it’s trying to say either. But:

Look at table 1 in para 8.4 - control L gives High-Z at the output.


True, and that line of Table 1 is the most recent item on the revision history. However, the second paragraph of the intro (Sec 3, pg 1) and paragraph 8.1 (pg 14) are consistent in their description of the outputs; that is, tied to either Vdd when Vc is high, or to Vss when Vc is low. I presume "high" means Vc = Vdd and "low" means Vc = Vss; that's certainly what many of the diagrams suggest.

If, as Bob says, the NTSC signal we're working with is DC biased and exists entirely above 0V, then we can use a single supply and it shouldn't be an issue. That will simplify the schematic, shrink the BoM and ease the layout.

There are apparent errors elsewhere in the datasheet as well. Fig 5 (pg 9, Typical ON Characteristics) includes a wiring diagram that shows -- as you would expect -- Vc = Vdd. However, Fig 9 (pg 10) depicts the test setup for this measurement and it shows Vc = Vss = -5V. Then Fig 10 (pg 11, OFF Switch Input or Output Leakage) shows exactly the same setup, with Vc = Vss. Apparently the outputs can be ON or OFF with the control high or low. This must be a quantum device!

This datasheet is a mess.

I guess we stand by for results of Bob's test with a sample camera, and Wade's test with multiple cameras through a switch...

Eric


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