8.1.2 ADC Driver and Reference Buffer

Figure 8-3 shows the use of a TLVx365 op amp as a SAR ADC input and reference pin driver. Sensors, which

are used for interfacing with the physical environment, exhibit high output impedance and cannot drive SAR ADC

inputs directly. TLVx365 exhibits a very low-input bias current of 20 pA (maximum), hence does not load these

high-output impedance sensors. A wide-GBW amplifier connected to the output of these sensor is needed to

charge the switching capacitors at the SAR ADC input and to settle fast, to the required accuracy, within the

given acquisition time.

The ADC core draws transient current from the reference input during the conversion (digitization) phase, which

needs to be driven with a wide-GBW amplifier to offer fast settling and maintain a stable reference voltage for

superior digitization performance. The TLVx365 devices reference buffer is used in a composite loop with the

OPA378 precision amplifier due to limitations in precision performance of wide-GBW amplifiers. The precision

amplifier maintains low-offset output, whereas the TLVx365 devices provide the output drive and fast-settling

performance.

ADS7945

14-bit

2 MSPS

R

S

+

–

5V

AVDD

DVDD

5V

5V

TLVx365

+

–

C

CB

5V

TLVx365

VREF

R

FILT

C

FILT

+

–

5V

OPA378

Reference

5V

R

F

R

G

Z

OUT

Sensor

Figure 8-3. TLVx365 as a SAR ADC Driver

8.1.3 Overdrive Recovery Performance

The TLVx365 family exhibits expectional overdrive recovery when the output is driven well beyond V+ or V–

supplies. When configured in a low-side current sensing configuration as shown in Figure 8-4, the output of the

opamp (TLVx365) if often driven to or below GND due to the groundbounce at the power GND or due to the ≤ 0

A current being measured across shunt resistance RSH. TLVx365 shows the ability to recover from an overdrive

event in < 100 ns. Figure 8-5 shows the comparison of the overdrive recovery performance of TLVx365 and

other popular opamps in the same catergory.

+

–

1 k

1 k

10 k

10 k

RSH

Switching

Circuit

ISH

VOUT

3.3V

DUT

ADS7056

+

–

3.3V

TLV3201

VTH

MCU

Interrupt

Digital I/O

300V

3.3V

GND

Figure 8-4. Low Side Current Sensing Application

Circuit

Time (200 ns/div)

-1

0

1

2

3

4

5

OPA365

OPA836

OPA607

TLV365

Gain = 10 V/V. VOUT driven to (V–) – 1V.

Figure 8-5. TLVx365 Overdrive Recovery

TLV365

SBOSAA8 – DECEMBER 2022

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