For systems which have different filter parameters or require specific system optimization, such as minimizing

the system noise, an alternative device may be desired. A list of recommended alternatives can be found in

Table 8-1.

Table 8-1. Alternative Devices

FEATURES

PRODUCT

Low-power, 10-MHz FET input industrial op amp

OPA140

2.2-nV/√ Hz, low-power, 36-V op amp in SOT-23 package

OPA209

Low-noise, high-precision, 22-MHz, 4-nV/√ Hz JFET-input op amp

OPA827

OPA376

Low-power, precision, CMOS, rail-to-rail input/output, low-offset, low-bias op amp

OPA191

Software tools are readily available to simplify filter design. WEBENCH® Filter Designer is a simple, powerful,

filter designs using a selection of TI operational amplifiers and passive components from TI's vendor partners.

Available as a web based tool from the WEBENCH Design Center, WEBENCH Filter Designer allows designers

to design, optimize, and simulate complete multistage active filter solutions within minutes.

8.2.3 Application Curve

Frequency (Hz)

-60

-40

-20

0

20

100

1k

10k

100k

1M

Figure 8-2. OPAx145 Second-Order, 25-kHz, Chebyshev, Low-Pass Filter

8.3 System Examples

8.3.1 16-Bit, 100-kSPS, Fully Differential Transimpedance Imaging and Measurement

The OPAx145 are used in a differential transimpedance (I-V) measurement application capable of driving the

ADS8867, a 16-bit, microPower, truly-differential ADC, at its maximum conversion rate of 100 kSPS with an

acquisition time of 1200 ns and conversion time of 8800 ns. The first stage supports a forward bandwidth of

493.5 kHz with 100 kΩ of transimpedance gain, enabling the photodiode to fully charge and settle to ±38 µV

(±1/2 LSB on 5-V ADC reference voltage) within the conversion time of the ADC. The differential nature of the

system provides several advantages such as double the transimpedance gain compared to a single-ended

system, improved signal-to-noise ratio, easy interfacing to high-precision, fully-differential ADCs, and additional

protection against inductively-coupled noise and interference. Additionally, capacitively-coupled common-mode

The second stage provides the reverse bandwidth required for settling to 16-bit accuracy after the internal

sampling capacitor of the successive-approximation-register (SAR) ADC is connected to the second stage. The

two OPAx145 amplifiers in the second stage are configured as buffers for maximum closed-loop bandwidth, and

their stability is optimized using R3, C3 and R4, C4 by creating a snubber that reduces the open-loop output

impedance (see Figure 6-26). C5 and C6 are provided as a charge reservoir for the internal sampling capacitor

of the ADC, and R5 and R6 are tuned to optimize the phase margin of the second stage to drive the output

OPA145,, OPA2145

SBOS427E – JUNE 2017 – REVISED OCTOBER 2020

www.ti.com

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