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I have a question about the input buffer of the PCM4202 ADC. On page 22 of the PCM4202 datasheet, they describe an input buffer. It's pretty standard with gain and filtering. However, the ADC requires the input to centered at +3V so it uses a COM pin on the op amp (OPA1632). However, this COM output from the PCM4202 needs to be buffered which makes sense. What doesn't make sense is why they use an OPA227, a high end expensive audio op amp. To me, that seems like a job for a ua741. Digging in to the OPA227 datasheet, I see a VERY low input offset voltage (+-5uV). Is this why they chose that op amp? What issues could arise if using a ua741 (since ua741s can have a high input offset voltage)?

Note that I have access to OPA2227s, which are 2 channel OPA227s with the same functionality excluding offset trim, and ua741s. However, I don't currently have the PCM4202 or the OPA1632.

Links are provided below of the datasheets:

http://www.ti.com.cn/cn/lit/ds/symlink/opa227.pdf http://www.ti.com/lit/ds/symlink/pcm4202.pdf http://www.ti.com/lit/ds/symlink/opa1632.pdf

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  • \$\begingroup\$ Do you have the large voltage rails that a 741 takes? Do you need the bandwidth? Do you care about input bias currents? \$\endgroup\$ – W5VO Apr 19 '16 at 21:41
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    \$\begingroup\$ Nothing calls for using an obsolete 741 any more \$\endgroup\$ – Scott Seidman May 5 '16 at 12:17
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TI does not specify anything about COM output load performance. May be the reason for use Low noise low input bias current OPA227 is the high impedance of COM output. May be they care about the noise injection via COM voltage.

So - may be general purpose OP AMP would be ok. May be, you get some small troubles with it.

You can consider AD8675, it is almost as good as OPA227 but much cheaper.

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  • \$\begingroup\$ Actually looking at it, a 2 channel OPA2227 is only a little more expensive than the AD8675. So the OPA2227 would be cheaper in the end. But I see similar performance specs. I'm not worried about the price of the op amp. I was just kind of curious about why they chose to use a high performance audio op-amp instead of a general purpose one. But you brought up some valid points. \$\endgroup\$ – Caleb Merchant Apr 20 '16 at 0:01
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The reason for the precision (ultra-low offset) op-amp is to ensure that the buffered COM pin remains as close to the 3.0 V as possible. Otherwise, there's a DC offset applied to the signal feeding the converter input, and that affects headroom. At or near a full-scale input, you'll hit either the upper or lower maximum, not both simultaneously.

The AD8675 is probably a good choice here. Precision op-amps are always more expensive than general-purpose types.

Also, be aware that there is a 30k/30k voltage divider on the OPA1632's VCM input. When left unconnected, this biases the VCM input to 0 V (a good thing), but if you drive it as suggested by the PCM4202 data sheet (Figure 13), that is, PCM4202 VCOM driving the buffer amp, and the buffer output driving OPA1632 VCM through a 1k resistor, the OPA1632 output symmetry and common-mode voltage both shift. That's the effect of the 1k resistor on the 30k/30k divider. So, instead, just drive the OPA1632 VCM pin from the buffer op-amp directly.

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