differences between shunt and series voltage references?

[Michael Noone]

Hi - I need a 4.096 voltage reference for an ADC. I'd like it to be as
accurate as possible, naturally. I've been looking at the various 4.096
voltage references (Maxim alone makes about 45) - and I've noticed that
they for the most part can be grouped as either shunt or as series voltage
references. From what I can tell a shunt V reference is just a zener diode
of the rated voltage. A series V reference is just a voltage regulator. Is
this right? Does one have a major advantage over the other? Thanks for your
help!

-Michael Noone

 

[Guy Macon]

Hi - I need a 4.096 voltage reference for an ADC. I'd like it to be as
accurate as possible, naturally. I've been looking at the various 4.096
voltage references (Maxim alone makes about 45) - and I've noticed that
they for the most part can be grouped as either shunt or as series voltage
references. From what I can tell a shunt V reference is just a zener diode
of the rated voltage. A series V reference is just a voltage regulator. Is
this right? Does one have a major advantage over the other? Thanks for your
help!

Look here:
Understanding Voltage-Reference Topologies and Specifications
[ http://pdfserv.maxim-ic.com/en/an/AN719.pdf ]

 

[Spehro Pefhany]

Hi - I need a 4.096 voltage reference for an ADC. I'd like it to be as
accurate as possible, naturally. I've been looking at the various 4.096
voltage references (Maxim alone makes about 45) - and I've noticed that
they for the most part can be grouped as either shunt or as series voltage
references. From what I can tell a shunt V reference is just a zener diode
of the rated voltage. A series V reference is just a voltage regulator. Is
this right? Does one have a major advantage over the other? Thanks for your
help!

-Michael Noone

Aside from series vs. shunt, some ADCs are quite demanding as far as
the reference output impedance goes.

This is a pretty accurate (0.05%) and stable (and low current)
reference for the price:

http://www.analog.com/UploadedFiles/Data_Sheets/564838270REF19x_h.pdf



Best regards,
Spehro Pefhany

 

[John Popelish]

Hi - I need a 4.096 voltage reference for an ADC. I'd like it to be as
accurate as possible, naturally. I've been looking at the various 4.096
voltage references (Maxim alone makes about 45) - and I've noticed that
they for the most part can be grouped as either shunt or as series voltage
references. From what I can tell a shunt V reference is just a zener diode
of the rated voltage. A series V reference is just a voltage regulator. Is
this right?

Yes.

Does one have a major advantage over the other? Thanks for your

help!

You have to compare the specs for noise and disturbance rejection.
The series reference often uses less current than a shunt reference
fed by just a resistor, if the raw voltage varys over a wide range.

 

[Robert Baer]

Hi - I need a 4.096 voltage reference for an ADC. I'd like it to be as
accurate as possible, naturally. I've been looking at the various 4.096
voltage references (Maxim alone makes about 45) - and I've noticed that
they for the most part can be grouped as either shunt or as series voltage
references. From what I can tell a shunt V reference is just a zener diode
of the rated voltage. A series V reference is just a voltage regulator. Is
this right? Does one have a major advantage over the other? Thanks for your
help!

-Michael Noone

I strongly suggest you look for *any* other brand; Maxim has the bad
habit of advertising vaporware.

 

[Ban]

Aside from series vs. shunt, some ADCs are quite demanding as far as
the reference output impedance goes.

This is a pretty accurate (0.05%) and stable (and low current)
reference for the price:

http://www.analog.com/UploadedFiles/Data_Sheets/564838270REF19x_h.pdf



Best regards,
Spehro Pefhany

But the dropout voltage is so high, it cannot be fed from the +5V rail, here
is a brand new reference REF3240
http://focus.ti.com/docs/prod/folde...P=hpa_dc_general&HQS=NotApplicable+PR+sc05144
it needs only 100mV for an output current of 8mA, and just 1mV if the
current is 1uA.

 

[PeteS]

Just to add another option...

I also like the National LM4120 for A-D references. I am using this as
a reference for an existing A-D subsystem (not high speed, I must admit
- system voltage monitoring in this application).

I am using the LM4120AIM5-4.1 which has the following specs:

180mV dropout at 1mA (worst case, typical = 120mV)
+/-5mA sink OR source
50ppm tempco
Output noise voltage 20uV p-p 0.1Hz - 10Hz, 36uV p-p 10Hz - 10kHz
Zo < 1 ohm for F < 10kHz ( at Iout = 1mA)

Comes in a nice SOT23-5 package.

Cheers

PeteS

 

[Michael Noone]

I strongly suggest you look for *any* other brand; Maxim has the
bad
habit of advertising vaporware.

Do you mean that they advertise alot of products that they never bring to
market? I noticed they had a couple really nice SOT-23 V-regs - and then I
realized that I couldn't find a distributor for *any* of the accurate ones.
I also noticed they don't have samples available for these parts - so it
seems to me that any parts that they have samples available for
available... Thus I was lookin at the Maxim MAX6126:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3623

..06% initial accuracy and it comes in a 8-uMAX package. (it says .02% on
the above page but the 4.096 8-uMAX part is .06% if you look at pg. 16 of
its datasheet) It does need a couple external components though (some small
capacitors). It also seems to be available as a sample - though I couldn't
find any distributor that stocked it. This is not a big deal as a total of
6 of these units will be made. I have made a sample request of this part -
I guess I'll have to see if it actually comes.

Does that look like a good choice?

-Michael

 

[Jonathan Kirwan]

<snip>
Aside from series vs. shunt, some ADCs are quite demanding as far as
the reference output impedance goes.
<snip>

I wanted to emphasize this. I've had to follow a reference with an
op-amp.

Jon

 

[Michael Noone]

Hi - I need a 4.096 voltage reference for an ADC. I'd like it to be as
accurate as possible, naturally. I've been looking at the various
4.096 voltage references (Maxim alone makes about 45) - and I've
noticed that they for the most part can be grouped as either shunt or
as series voltage references. From what I can tell a shunt V reference
is just a zener diode of the rated voltage. A series V reference is
just a voltage regulator. Is this right? Does one have a major
advantage over the other? Thanks for your help!

Look here:
Understanding Voltage-Reference Topologies and Specifications
[ http://pdfserv.maxim-ic.com/en/an/AN719.pdf ]

Thanks - that was very helpful. I had been looking for something like that
and just couldn't find it for some odd reason.

-Michael

 

[Robert Baer]

Do you mean that they advertise alot of products that they never bring to
market? I noticed they had a couple really nice SOT-23 V-regs - and then I
realized that I couldn't find a distributor for *any* of the accurate ones.
I also noticed they don't have samples available for these parts - so it
seems to me that any parts that they have samples available for
available... Thus I was lookin at the Maxim MAX6126:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3623

.06% initial accuracy and it comes in a 8-uMAX package. (it says .02% on
the above page but the 4.096 8-uMAX part is .06% if you look at pg. 16 of
its datasheet) It does need a couple external components though (some small
capacitors). It also seems to be available as a sample - though I couldn't
find any distributor that stocked it. This is not a big deal as a total of
6 of these units will be made. I have made a sample request of this part -
I guess I'll have to see if it actually comes.

Does that look like a good choice?

-Michael

If you do not mind waiting for a long time or possibly forever...

 

[PeteS]

Maxim does not have the best reputation for delivering on their
'products', especially in their reference stuff.

Even though they have claimed to me in the past that they have 'changed
their ways' and will ship even if you don't want to buy 100k units of
something, I have found their delivery very spotty. That can be a big
problem if your system fundamentally relied on the function of the
device you want from them, and they blithely quote 8-14 weeks lead
time, when their reps etc. have promised there is sufficient stock on
hand for the build you wanted to schedule in 3 weeks.

I always try and find an alternative (or multiply sourced) part when I
see Maxim on the datasheet / suggested list.

Cheers

PeteS