LMP7717 datasheet(34/56 Pages) TI

34 / 56 page

8.2.2 Transimpedance Amplifier

A common application for a high-gain-bandwidth voltage-feedback op amp is to amplify a small photodiode

current from a capacitive detector. Figure 8-7 shows the front-page transimpedance circuit with more detail.

Here, a fixed –0.23 negative voltage generator (LM7705) is used on the negative supply to make sure the output

has adequate headroom when the output is at 0 V. The transimpedance stage is designed here for a 2.4

‑MHz

flat (Butterworth) response while a simple RC post-filter band-limits the broadband noise and sets the overall

bandwidth to 1 MHz. The requirements for a high-dynamic-range transimpedance (or charge) amplifier include

the very low input voltage noise intrinsic to a decompensated device like the OPA838. The noise gain over

frequency for this type of circuit starts out at unity gain, and then begins to peak with a single zero response.

This peaking is due to the pole formed in the feedback by the feedback resistor and the total capacitance on the

inverting input. That noise gain response is flattened out at higher frequencies by the feedback capacitor value to

be the 1 + CS / CF capacitor ratio. This noise gain is normally a very high, allowing the decompensated OPA838

to be applied to this application. The noise gain is intentionally peaked to a high value in this application;

therefore, the very low input voltage noise (1.8 nV/√Hz) of the OPA838 improves dynamic range.

RF100 k

CF1.0 pF

Bias Current

Cancellation

and R-noise

filtering

2.4-Mhz

Butterworth

1-MHz

Low Pass

VOUT

U1 OPA838

VM1

IDIODE

VEE

VCC

VEE

VCC

V1 3 V

LM7805 ± 230 mV

R1 20

R2 73.2

Figure 8-7. 100-kΩ Wide Bandwidth Transimpedance Design

8.2.2.1 Design Requirements

To implement a controlled frequency response transimpedance design, set the transimpedance stage amplifier

bandwidth higher than a controlled post RC filter. This allows variation in the source capacitance and amplifier

gain bandwidth product with less overall bandwidth variation to the final output. In this example design:

• Assume a nominal source capacitance value of 100 pF. This normally comes from the capacitance versus

reverse bias plot for the photodiode. No reverse bias is illustrated in Figure 8-8, but the current source is

typically a back biased diode with a negative supply on the anode and the cathode connected to the op amp

inverting input. In this polarity, the signal current sinks into the diode and raises the op amp output voltage

above ground.

• For the best dc precision, add a matching resistor on the noninverting input to reduce the input bias current

error to IOS × RF. This resistor adds to the input voltage noise; TI recommends bypassing that resistor with

as large as a capacitor as required to roll off resistor noise. This capacitor has a relatively low frequency self

resonance that interacts with the input stage and can impair stability. Add a small series 20

‑Ω resistor from

the capacitor into the noninverting input to de-Q the resonant source impedance without adding too much

noise.

• Set the feedback capacitor to achieve the desired frequency response shape.

• Add a post RC filter to control the overall bandwidth to 1 MHz. In this example, a 2.2

‑nF capacitor allows

a low 73.2

‑Ω series resistor. When driving a sampling ADC (like a SAR), this combination helps reduce the

sampling glitch and speed settling time.

OPA838

SBOS867D – AUGUST 2017 – REVISED SEPTEMBER 2024

www.ti.com

Submit Document Feedback

Product Folder Links: OPA838

LMP7717 Datasheet(PDF) 34 Page - Texas Instruments

LMP7717 Datasheet(HTML) 34 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Datasheet Download

Link URL