REF3425: output capacitor value

Alen Barisic

Intellectual 1300 points

Part Number: REF3425
Other Parts Discussed in Thread: OPA1612, OPA211

Hello,

the datasheet mentions a „Stable output capacitor value“ of 0.1µF - 10µF.

Are there any possible issues that could occur when 100µF are used?

Thanks,

Alen

over 3 years ago

0 Marcoo Z over 3 years ago

TI__Mastermind 19370 points

Hi Alen,

The 100uF is too large and can cause instability on the output buffer. This instability can make the output to deviate from the intended voltage or oscillate.

I would recommend to use an output external op amp buffer to drive 100uF.

What is the need for the 100uF cap?

0 Alen Barisic over 3 years ago in reply to Marcoo Z

TI__Intellectual 1300 points

Hi Marcoo,

the used ADC needs 100uF on the REF input.

Which OpAmp buffer would you suggest to drive the 100uF (Single Supply 5V)?

Thanks,

Alen

0 Marcoo Z over 3 years ago in reply to Alen Barisic

TI__Mastermind 19370 points

Hi Alen,

Is this a single ADC REF input? I have not seen a value so high.

Can you share the ADC name or uC name? If its NDA feel free to take this offline and email us.

0 Alen Barisic over 3 years ago in reply to Marcoo Z

TI__Intellectual 1300 points

Hi Marcoo,

It is the AD7767.

Regards,

Alen

0 Marcoo Z over 3 years ago in reply to Alen Barisic

TI__Mastermind 19370 points

Hi Alen,

I have assigned this to the precision op amp group who can give you strong recommendations.

0 Alen Barisic over 3 years ago in reply to Marcoo Z

TI__Intellectual 1300 points

Hi Marcoo,

looking forward to it.

Thanks,

Alen

0 Marek Lis over 3 years ago in reply to Alen Barisic

TI__Guru 60570 points

I would recommend to use our lowest noise op amp: OPA211 or OPA1612, to buffer the reference voltage. You will have to use specific compensation to drive large cap load.

0 Thomas Kuehl over 3 years ago

TI__Guru* 93105 points

Hi Allen,

I can see from the AD7767 datasheet that the reference they are using it will drive a 100 uF load directly connected to the VREF+ pin. The inputs and REF pins of an ADC do represent a dynamic load where the current required is transient in nature and varies with the conversion cycles. Therefore, if you add an op amp buffer to the VREF+ it usually isn't a simple matter because it must be have sufficient speed and transient current capability to drive the pin. Otherwise, it is possible to degrade the performance of the converter by adding extra circuitry.

If you need an op amp buffer that can drive 100 uF directly at its output, then we should be able to come up with a suggestion for one and a compensation scheme so that it remains stable. However, if you are trying to optimize the converter performance and have the 100 uF load at the VREF+ pin that is a different, more involved undertaking.

TI's converted products e2e forum has the highest level of understanding about driving converter pins for maximum performance. They certainly focus on TI's converter products, but may have come across the AD7767 and have some idea how to best drive its VREF+ pin. On the other hand, I do wonder why you wouldn't apply the reference that the AD7767 shows in its datasheet as it appears the solution already exists without adding additional op amps.

Regards, Thomas

Precision Amplifiers Applications Engineering