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Martin | 8 May 18:44

[EE] bipolar digital pot

I have an analog circuit that has a (normally) negative polarity DC
feedback signal. I'd like to change the gain of the circuit digitally,
so I'd like to use a digital pot. The problem is that a digital pot can
only take signals between VEE and VDD - most usually probably 0v to 5v.

My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.
Has anyone encountered a digital pot that can either take a 10v supply,
or has level shifters on the digital lines? I think I might be able to
use a CD4052 with discrete resistors, but then it's a 16 pin chip I have
to put on the board.

Any ideas?

-
Martin
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Vasile Surducan | 8 May 21:29

Re: [EE] bipolar digital pot

On 5/8/08, Martin <martin <at> nnytech.net> wrote:
> I have an analog circuit that has a (normally) negative polarity DC
> feedback signal. I'd like to change the gain of the circuit digitally,
> so I'd like to use a digital pot. The problem is that a digital pot can
> only take signals between VEE and VDD - most usually probably 0v to 5v.
>
> My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.

So? Who stops you to supply your potentiometer driver (assuming it's a
PIC) between 0 and -5V ?

> Has anyone encountered a digital pot that can either take a 10v supply,
> or has level shifters on the digital lines? I think I might be able to
> use a CD4052 with discrete resistors, but then it's a 16 pin chip I have
> to put on the board.
>
> Any ideas?
>
> -
> Martin
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M. Adam Davis | 8 May 21:33

Re: [EE] bipolar digital pot

You have a few options outside your chosen solution, most of which can
be explored via this google search:
http://www.google.com/search?q=digital+gain+control

First, you might look at amplifiers with built int digital gain
control, such as these Linear Technology parts:
http://www.linear.com/pc/viewCategory.jsp?navId=H0,C1,C1154,C1009,C1121
If the link doesn't work, navigate to
Home  >  Products  >  Signal Conditioning  >  Operational Amplifiers
(Op Amps)  >  Programmable Gain Amplifiers

Second, you might look into CMOS digital switches - some have a 20v range:
http://schematics.blogspot.com/2005/01/digital-gain-control-of-opamp.html

Of course, if you really want to kludge something together, tie a
servo to a pot...  ;-D

Good luck!

-Adam

On 5/8/08, Martin <martin <at> nnytech.net> wrote:
> I have an analog circuit that has a (normally) negative polarity DC
> feedback signal. I'd like to change the gain of the circuit digitally,
> so I'd like to use a digital pot. The problem is that a digital pot can
> only take signals between VEE and VDD - most usually probably 0v to 5v.
>
> My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.
> Has anyone encountered a digital pot that can either take a 10v supply,
> or has level shifters on the digital lines? I think I might be able to
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alan smith | 8 May 21:57
Favicon

[EE] pulling power from the telco line

I'm reviewing a potential project for a client, where his device is inline with the telco tip/ring lines.

  Essentially is a speed dial/memory type where it takes the phone line when it goes the phone goes off hook and
then returns it back to the telco line.

  So, he will want to have it pass Part 65 (thats telco...or is it 68?) and so there is the isolation issues, but
he also wants to pull power from the telco line to run the micro and such.

  Now, we all know that Ma Bell hates it when something is attached to the telco line, other than her stuff.  So if
a device is pulling power off the line, will it pass FCC or get dumped right away.

I want to squash his dreams right away rather than get down the road and then find its not going to pass.

       
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David VanHorn | 9 May 03:10

Re: [EE] pulling power from the telco line

>  So, he will want to have it pass Part 65 (thats telco...or is it 68?) and so there is the isolation issues, but
he also wants to pull power from the telco line to run the micro and such.

Part 68

> I want to squash his dreams right away rather than get down the road and then find its not going to pass.

You're allowed a few microamps or so.  You can also draw power when
you're off-hook, or when the line is ringing, to charge up a supercap,
if you can do it without upsetting the impedances.  Constant current
sources are your friend.
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Robert Ammerman | 9 May 04:33

Re: [EE] pulling power from the telco line

From: "David VanHorn" <microbrix <at> gmail.com>
> You're allowed a few microamps or so.  You can also draw power when
> you're off-hook, or when the line is ringing, to charge up a supercap,
> if you can do it without upsetting the impedances.  Constant current
> sources are your friend.

To make clear what David said:

When the line is on-hook you can draw _very_little current (otherwise the 
Telco thinks you have gone off hook). This is a matter of microamps. The 
good news is the on-hook (open loop) voltage is pretty high, somewhere 
around 48V.

When the line is off-hook you can draw quite a bit more current (but of 
course the voltage will be quite a bit lower).

You can also extract power from the ringing voltage (about 20Hz AC, maybe 
100V or so). The source impedance is pretty high and the Telco only allows a 
certain load on the line (measured in "Ringer Equivalences").

---- Bob Ammerman
RAm Systems

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Harold Hallikainen | 9 May 21:34

Re: [EE] pulling power from the telco line

The on-hook current limits are VERY low. Here are the US limits from
http://part68.org/SecureDocuments/TIA-968-A-Final.pdf .

4.7.2.1 On-hook resistance, metallic and longitudinal (up to 100 V dc).
The on-hook dc resistance between the tip and ring conductors of a loop
start interface, and between each of the tip and ring conductors and earth
ground, shall be greater than 5 M&#8486; for all dc voltages up to and
including 100 V.

4.7.2.2 On-hook resistance, metallic and longitudinal (100 V to 200 V dc).
The on-hook dc resistance between tip and ring conductors of a loop start
interface, and between each of the tip and ring conductors and earth
ground shall be greater than 30 k&#8486; for all dc voltages between 100
and 200 V.

Harold

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David VanHorn | 9 May 22:30

Re: [EE] pulling power from the telco line

> 4.7.2.1 On-hook resistance, metallic and longitudinal (up to 100 V dc).
> The on-hook dc resistance between the tip and ring conductors of a loop
> start interface, and between each of the tip and ring conductors and earth
> ground, shall be greater than 5 M&#8486; for all dc voltages up to and
> including 100 V.

So at 48V that's almost 10uA, which is enough for memory retention and
limited operation.

> 4.7.2.2 On-hook resistance, metallic and longitudinal (100 V to 200 V dc).
> The on-hook dc resistance between tip and ring conductors of a loop start
> interface, and between each of the tip and ring conductors and earth
> ground shall be greater than 30 k&#8486; for all dc voltages between 100
> and 200 V.

I'm not sure under what condition this applies, both of these can't be
true in the same case.
But 30k allows 1.6mA

Proviso:  Although on-hook voltage is nominally 48, that will range as
high as 56 in real life.
24V is not uncommon, and I have seen 12V on-hook.

Memory retention by microcontrollers is of course commonly done in
phones with speed dial memories.
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Harold Hallikainen | 10 May 00:51

Re: [EE] pulling power from the telco line


>> 4.7.2.1 On-hook resistance, metallic and longitudinal (up to 100 V dc).
>> The on-hook dc resistance between the tip and ring conductors of a loop
>> start interface, and between each of the tip and ring conductors and
>> earth
>> ground, shall be greater than 5 M&#8486; for all dc voltages up to and
>> including 100 V.
>
> So at 48V that's almost 10uA, which is enough for memory retention and
> limited operation.
>
>> 4.7.2.2 On-hook resistance, metallic and longitudinal (100 V to 200 V
>> dc).
>> The on-hook dc resistance between tip and ring conductors of a loop
>> start
>> interface, and between each of the tip and ring conductors and earth
>> ground shall be greater than 30 k&#8486; for all dc voltages between 100
>> and 200 V.
>
> I'm not sure under what condition this applies, both of these can't be
> true in the same case.
> But 30k allows 1.6mA

I think the DC voltages between 100 and 200 is MAYBE during ringing (but,
of course, that's AC on top of DC). I don't know when else it would apply.

Harold

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Robert Ammerman | 12 May 00:56

Re: [EE] pulling power from the telco line

> I think the DC voltages between 100 and 200 is MAYBE during ringing (but,
> of course, that's AC on top of DC). I don't know when else it would apply.
>
> Harold

I think that they don't want significant current to flow during some kinds 
of 'fault' conditions (like a short to a power line maybe?)

This is probably here because many Telco-interfaced devices include MOVs or 
similar devices which can present a very low impedance to a voltage spike.

I am guessing that he milli-amp level current allowed at 200V is still below 
the real trip current for an off-hook condition.

-- Bob Ammerman
RAm Systems

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David VanHorn | 12 May 04:24

Re: [EE] pulling power from the telco line

> I am guessing that he milli-amp level current allowed at 200V is still below
> the real trip current for an off-hook condition.

You have to draw at least 20mA to guarantee being detected as off-hook.
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Robert Ammerman | 12 May 05:04

Re: [EE] pulling power from the telco line

>>Bob Ammerman:
>> I am guessing that he milli-amp level current allowed at 200V is still 
>> below
>> the real trip current for an off-hook condition.

>From: "David VanHorn" <microbrix <at> gmail.com>
> You have to draw at least 20mA to guarantee being detected as off-hook.

I should have said below the 'maximum guaranteed stay-on-hook current.'

-- Bob Ammerman
RAm Systems 

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alan smith | 12 May 15:28
Favicon

Re: [EE] pulling power from the telco line

Thanks for all the information.  I'd prefer to leave the power in a battery, and if it will run for 6 months or a
year on a couple of AA batteries, then that should actually be a little easier to design around.

Robert Ammerman <rammerman <at> verizon.net> wrote:  >>Bob Ammerman:
>> I am guessing that he milli-amp level current allowed at 200V is still 
>> below
>> the real trip current for an off-hook condition.

>From: "David VanHorn" 
> You have to draw at least 20mA to guarantee being detected as off-hook.

I should have said below the 'maximum guaranteed stay-on-hook current.'

-- Bob Ammerman
RAm Systems 

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Justin Richards | 18 May 09:00

Re: [EE] pulling power from the telco line

In Australia there was a Blue Coin Phone that used the telco line
(effectively a normal POTS line with metering pulses) to supply all
the power to operate the coin mechanism.  Solenoids etc.

After it was installed for the first time it would take a little
while, about 5 mins to charge up the capacitor before the phone could
be tested.

It had all the components that a pay phone would need.  Ie
Microprocessor, display, coin mech etc.

Cheers Justin

On Mon, May 12, 2008 at 9:28 PM, alan smith <micro_eng2 <at> yahoo.com> wrote:
> Thanks for all the information.  I'd prefer to leave the power in a battery, and if it will run for 6 months or a
year on a couple of AA batteries, then that should actually be a little easier to design around.
>
> Robert Ammerman <rammerman <at> verizon.net> wrote:  >>Bob Ammerman:
>>> I am guessing that he milli-amp level current allowed at 200V is still
>>> below
>>> the real trip current for an off-hook condition.
>
>
>>From: "David VanHorn"
>> You have to draw at least 20mA to guarantee being detected as off-hook.
>
> I should have said below the 'maximum guaranteed stay-on-hook current.'
>
> -- Bob Ammerman
> RAm Systems
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Mark Rages | 8 May 22:06

Re: [EE] bipolar digital pot

On Thu, May 8, 2008 at 11:45 AM, Martin <martin <at> nnytech.net> wrote:
> I have an analog circuit that has a (normally) negative polarity DC
> feedback signal. I'd like to change the gain of the circuit digitally,
> so I'd like to use a digital pot. The problem is that a digital pot can
> only take signals between VEE and VDD - most usually probably 0v to 5v.
>
> My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.
> Has anyone encountered a digital pot that can either take a 10v supply,
> or has level shifters on the digital lines? I think I might be able to
> use a CD4052 with discrete resistors, but then it's a 16 pin chip I have
> to put on the board.
>
> Any ideas?
>

I assume the digital lines are 0->+5 V.

The digital pots I've worked with use three lines for their digital
interface.  These three are all inputs.

You can use a PNP transistor for a level shifter.

Instead of ASCII art, I'll try to describe the connection:  The PIC
output  is connected through a 10k resistor to the emitter, the base
is connected to ground, and the collector is connected to the digital
pot's input.  Also connected to the collector is a 100k pulldown
resistor to -5V.

Now when the PIC output is low, no current flows and the digital pot's
input sees -5V (low)
(Continue reading)

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Vasile Surducan | 9 May 06:07

Re: [EE] bipolar digital pot

On 5/8/08, Mark Rages <markrages <at> gmail.com> wrote:
> On Thu, May 8, 2008 at 11:45 AM, Martin <martin <at> nnytech.net> wrote:
> > I have an analog circuit that has a (normally) negative polarity DC
> > feedback signal. I'd like to change the gain of the circuit digitally,
> > so I'd like to use a digital pot. The problem is that a digital pot can
> > only take signals between VEE and VDD - most usually probably 0v to 5v.
> >
> > My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.
> > Has anyone encountered a digital pot that can either take a 10v supply,
> > or has level shifters on the digital lines? I think I might be able to
> > use a CD4052 with discrete resistors, but then it's a 16 pin chip I have
> > to put on the board.
> >
> > Any ideas?
> >
>
> I assume the digital lines are 0->+5 V.

If he is designing the whole stuff, you may assume digital lines are 0
-5V (if it's a clever designer) so he don't need any level shifter.

Vasle

>
> The digital pots I've worked with use three lines for their digital
> interface.  These three are all inputs.
>
> You can use a PNP transistor for a level shifter.
>
> Instead of ASCII art, I'll try to describe the connection:  The PIC
(Continue reading)

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Martin | 9 May 16:11

Re: [EE] bipolar digital pot

Vasile Surducan wrote:
> On 5/8/08, Mark Rages <markrages <at> gmail.com> wrote:
>> On Thu, May 8, 2008 at 11:45 AM, Martin <martin <at> nnytech.net> wrote:
>>> I have an analog circuit that has a (normally) negative polarity DC
>>> feedback signal. I'd like to change the gain of the circuit digitally,
>>> so I'd like to use a digital pot. The problem is that a digital pot can
>>> only take signals between VEE and VDD - most usually probably 0v to 5v.
>>>
>>> My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.
>>> Has anyone encountered a digital pot that can either take a 10v supply,
>>> or has level shifters on the digital lines? I think I might be able to
>>> use a CD4052 with discrete resistors, but then it's a 16 pin chip I have
>>> to put on the board.
>>>
>>> Any ideas?
>>>
>> I assume the digital lines are 0->+5 V.
> 
> If he is designing the whole stuff, you may assume digital lines are 0
> -5V (if it's a clever designer) so he don't need any level shifter.
> 
> Vasle
> 

If I'm clever, eh? First product I designed, I'm the only engineer
working on it - I'm lucky it works at all. Digital has to be ground
referenced.

The input opamp is an LMC6042AIN. I need the extremely low input bias
current spec. so I can't use a controlled-gain amplifier i.e. LTC6910.
(Continue reading)

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Sean Breheny | 12 May 05:06
Favicon

Re: [EE] bipolar digital pot

Hi Martin,

On Fri, May 9, 2008 at 10:11 AM, Martin <martin <at> nnytech.net> wrote:
>  The input opamp is an LMC6042AIN. I need the extremely low input bias
>  current spec. so I can't use a controlled-gain amplifier i.e. LTC6910.

Why can't you use the 6042 to buffer the signal and then use a
variable-gain amplifier after that?

Sean
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Martin | 12 May 14:25

Re: [EE] bipolar digital pot

Sean Breheny wrote:
> Hi Martin,
> 
> On Fri, May 9, 2008 at 10:11 AM, Martin <martin <at> nnytech.net> wrote:
>>  The input opamp is an LMC6042AIN. I need the extremely low input bias
>>  current spec. so I can't use a controlled-gain amplifier i.e. LTC6910.
> 
> Why can't you use the 6042 to buffer the signal and then use a
> variable-gain amplifier after that?
> 
> Sean

The input stage has a high dynamic range. We'll see if it works.
-
Martin
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Vasile Surducan | 12 May 10:36

Re: [EE] bipolar digital pot

On 5/9/08, Martin <martin <at> nnytech.net> wrote:
> Vasile Surducan wrote:
> > On 5/8/08, Mark Rages <markrages <at> gmail.com> wrote:
> >> On Thu, May 8, 2008 at 11:45 AM, Martin <martin <at> nnytech.net> wrote:
> >>> I have an analog circuit that has a (normally) negative polarity DC
> >>> feedback signal. I'd like to change the gain of the circuit digitally,
> >>> so I'd like to use a digital pot. The problem is that a digital pot can
> >>> only take signals between VEE and VDD - most usually probably 0v to 5v.
> >>>
> >>> My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.
> >>> Has anyone encountered a digital pot that can either take a 10v supply,
> >>> or has level shifters on the digital lines? I think I might be able to
> >>> use a CD4052 with discrete resistors, but then it's a 16 pin chip I have
> >>> to put on the board.
> >>>
> >>> Any ideas?
> >>>
> >> I assume the digital lines are 0->+5 V.
> >
> > If he is designing the whole stuff, you may assume digital lines are 0
> > -5V (if it's a clever designer) so he don't need any level shifter.
> >
> > Vasle
> >
>
> If I'm clever, eh? First product I designed, I'm the only engineer
> working on it - I'm lucky it works at all. Digital has to be ground
> referenced.
>
> The input opamp is an LMC6042AIN. I need the extremely low input bias
(Continue reading)

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Spehro Pefhany | 8 May 22:06

Re: [EE] bipolar digital pot

Quoting Martin <martin <at> nnytech.net>:

> I have an analog circuit that has a (normally) negative polarity DC
> feedback signal. I'd like to change the gain of the circuit digitally,
> so I'd like to use a digital pot. The problem is that a digital pot can
> only take signals between VEE and VDD - most usually probably 0v to 5v.
>
> My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.
> Has anyone encountered a digital pot that can either take a 10v supply,
> or has level shifters on the digital lines? I think I might be able to
> use a CD4052 with discrete resistors, but then it's a 16 pin chip I have
> to put on the board.
>
> Any ideas?

There do exist digital pots which will operate from +/-5V supplies, with
level shifting for the digital bits. AD is one supplier, but they tend
to be more expensive (and higher performance!) than the low end ones. There
might be other suppliers with similar products, but I'm not aware of
any offhand.

Eg. http://www.analog.com/UploadedFiles/Data_Sheets/AD5290.pdf

Best regards,
Spehro Pefhany
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