Hi all,
My question is about precision transimpedance amplifiers and, specifically, what properties of the op-amp dictate the drift characteristics of the TIA ?s output signal.
Suppose we take as an example a TIA made with the OPA847, which is not a st andard high precision amp and is instead chosen for its GBW. The gain resis tor is taken as 10k, and we have 100uA of photocurrent, for a voltage outpu t of 1V. Let?s also assume for a minute that the tempco of the feed back resistor is negligible. I?m interested in understanding the op
-amp specifications that are important for minimizing the drift of this 1V output.
The usual specification of interest for high precision amps is the offset v oltage, which is 100uV and exhibits a drift of 0.25uV/deg. The offset volta ge is of course gained up by the circuit?s noise gain. However, con sidering that the shunt resistance of the photodiode is infinite compared t o the 10k gain resistor, the noise gain is unity at DC. Thus, we have a 0.2
5uV/deg drift on the output. This works out to be 0.25ppm/deg, which is ver y good. Note that I don?t care about the output accuracy but rather that it doesn?t change.As noted in Jerald Graeme?s book, the offset voltage and its associ ated drift also appear across the photodiode and tickle its reverse biasing . This changes the dark current that is amplified that is amplified along w ith the photocurrent of interest. At a typical reverse bias of -5V, the dar k current of my standard photodiode is ~40pA and possesses a linearized bia s sensitivity of ~5pA/V. Assuming 100uA of photocurrent and the offset volt age drift of 0.25uV/deg, this effect contributes at less than the ppt level and is insignificant. It seems to be that offset voltage stability is not an issue, and the relat ive contribution of offset voltage drift can always be cured with additiona l transimpedance gain.
The next issue would be the input bias currents of the TIA. Since the OPA84
7 has a bipolar frontend, it exhibits larger bias currents and drifts. The non-inverting terminal of the amp is connected to ground via a resistor mat ching the gain resistor of the TIA, so we should only have to consider inpu t current offsets (0.1nA/deg). However, even if we ignore this matching and assume a worst-case scenario for the input bias current (19uA) and drift ( 15nA/deg), the drift contributes at the 15nA/deg/100uA = 150ppm/deg level .It seems to me that the op-amp properties of bias current drifts are only g ermane to low-light applications, i.e., photocurrents on the order of the b ias current offset drifts. Again, I?m not concerned about an overal l 100uV offset of the output signal. I just want to make sure that it does not change over time. The effects of voltage offset drift and bias current offset drift seem to be smaller than the tempco of the feedback resistor it self, e.g., 100ppm/deg for a ~1% resistor.
That being said, why do I always see app notes and articles in the scientif ic literature where people jump through all sorts of hoops to have TIA fron tends specifically designed to minimize this drift, including the use of ch opper-stabilized and zero-drift amps. I feel like my analysis is either wro ng or incomplete. What am I missing ?
Thanks in advance for your help and input !
-Jon