single-ended to differential?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Member
Joined 2007
Paid Member
Hi DIYs,

I have an IC with a differential analog input and I will be connecting a single-ended signal to this input.

My approach will be to ground the VIN- of the IC and connect the positive signal part of the single-ended input to VIN+ on the IC. And then connect the grounds.

Just want to hear if there is a better way....

Thanks,

Jesper
 
Member
Joined 2007
Paid Member
@anonymous

uhuh, seems I'm opening up for more than just the quick advice I was looking for.... ;)

As far as I remember from the theory I learned some years ago not connecting it in differential mode will cost 3 or 6 dB SNR...

Are you thinking of something else when you say
it would kind of defeat the purpose of using such a high end ADC
?

Best Jesper
 
Cauhtemoc said:
An opamp would be the simplest choice. Check page 23 of the PCM4202 datasheet.

Figure 14 on page 23 of the datasheet is a buffer for the common-mode reference voltage, not the inputs.

mako1138 said:
Note that the analog inputs have an absolute max range of -0.3V to (Vcc+0.3V). So if you grounded one input, the other would only be able to swing 0.6 Vpp, assuming a symmetrical audio signal. Is that enough for your purposes?

That would be -0.3V to 5.3V (assuming Vcc is 5V), not -0.3V to 0.3V.
 
mako1138 said:
I assumed that he would have no intervening circuitry, thus the signal would be centered at ground. Though I guess he could just use a coupling capacitor, since the inputs are biased to Vcc/2. There'd be a low-frequency cutoff in that case.

Another good reason for using the OPA1632 since it's outputs can be biased to Vcc/2 by connecting Vocm to Vcom of the ADC. There would be no need for coupling caps in this case and no associated low end cutoff.
 
theAnonymous1 said:
My apologies, I assumed you were mistakingly referring to the Vcom buffer because I had already mentioned the OPA1632 input buffer in post #4.

My appologies as well, I managed to miss post #4.

Anyway, to original poster, the OPA1632 was made for exactly this type of thing. It's the easiest, simplest and best solution you will find without going to the extremes of complexity.
 
theAnonymous1 said:


Another good reason for using the OPA1632 since it's outputs can be biased to Vcc/2 by connecting Vocm to Vcom of the ADC. There would be no need for coupling caps in this case and no associated low end cutoff.

Yeah, certainly the most straightforward solution is to follow the datasheet. I just wanted to advise the original poster of the consequences if he decided to go ahead and ground an input, per his original proposal.
 
Little bit of caution if you're using figure 13.

If the source impedance of the + and - inputs aren't the same, ie. if the - input is grounded (0 ohms) and the + input has a non-zero source impedance, you'll get a DC offset on the output.

Poke an op-amp buffer on the + input to get around this. A single OPA2134 or similar should suffice for both channels.
 
Member
Joined 2007
Paid Member
Thank you all for your suggestions and inputs ..... It clarified issues that I have to pay attention :)

To the end of this A/D project that I'm doing - any of you who knows how I can transfer the 2 channels of 24 bits/192 khz via USB to my laptop?

I'm looking for a straightforward chip solution that accepts the output from the PCM 4202 and converts it to USB in a way so that the laptop recognizes that a new USB device is present (with the characteristics necessary for audiosignal transmission).

My aim is to not have to do any programming ....

Any ideas?

Thanks Jesper
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.