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Design >> Analog Design >> PGA gain and UGB Specs
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Message started by packiaraj on Oct 5^th, 2006, 10:59pm

Title: PGA gain and UGB Specs
Post by packiaraj on Oct 5^th, 2006, 10:59pm

Hi,
For PGA to be used at the front-end of an ADC, OPAMP offset, its Input-referred noise, THD, SNR are important specs. As it's a continuous-time block how do we decide on the OPAMP's OPEN LOOP GAIN, 3dB Bandwidth, slew rate & settling time etc. Literally, How do we start with the design (Deriving specifications for OPAMP)? ::)

Thanks,
Packiaraj.V.

Title: Re: PGA gain and UGB Specs
Post by Sarig on Oct 6^th, 2006, 7:24am

Hi
Does this PGA is for a single slope ADC ?
In order to answer we need understand the probelm, this is the cool thing in Analog ;)
Explain how you plan to use it and I will answer you
Good luck
Erez

Title: Re: PGA gain and UGB Specs
Post by loose-electron on Oct 6^th, 2006, 2:47pm

Suggest:

Start by defining your input signal and output signal definitions. From there, you will get slew needs, linearity needs etc.

Go from there thru some behavioral models, and get your system working with some ABM (Analog Behavioral Model) structures.

As you go through that process, put less than ideal parameters into your op-amps and find out what you need to have.

Hope that helps,

Jerry

Title: Re: PGA gain and UGB Specs
Post by packiaraj on Oct 6^th, 2006, 11:21pm

Hi Erez and Jerry,
Thanks for your reply.

Yesterday, I thought of the following as the flow of design of OPAMP. Tell me if I go wrong anywhere.

The Preamp output goes to 13 Bit ADC...(which defines us the no. of time constants for settling which is=ln(2^N)=(~10) for N=13 and the gain error which is= 1/(AF) <1/[2^(N+1]=(~0.01%)), where A=Open-loop Gain of OPAMP and F is feedback factor.

The ADC's sampling time-window would tell us the settling time specification for the OPAMP, which is = (Time constant of the closed-loop OPAMP*no.of time constants). The Preamp's closed loop gain is defined by resistive feedback... (0 to 45dB insteps of 1.5dB).

From the resistive feedback we know what is feedback factor (F).

Find open-loop gain A from gain error in-equality 1/(AF) <1/[2^(N+1] --------(1)

Time constant of closed-loop OPAMP= (sampling time/ no. of time constants)

From sampling time and no. of time constants we can find the Time constant of the closed-loop OPAMP.

Time constant of closed-loop OPAMP= 1/{(1+A*F)*[W-3dB]} ---------- (2)

where W-3dB is -3dB angular frequency of the open-loop OPAMP

Substitute A found from (1) & F, in (2) to find W-3dB for the open-loop OPAMP.

Hence its UGB=(A*W-3dB) can be found.

As Jerry said Input & output signal swing would tell us how much is Slewing time & Linearity requirement.

Thanks,
Packiaraj.V.