8.3 System Example

8.3.1 Time Gain Control System for Ultrasound Applications

During an ultrasound send-receive cycle, the magnitude of reflected signal depends on the depth of penetration.

The ultrasound signal incident on the receiver decreases in amplitude as a function of the time elapsed since

transmission. The time gain control (TGC) system helps achieve the best possible signal-to-noise ratio (SNR),

even with the decreasing signal amplitude. When the image is displayed, similar materials have similar

brightness, regardless of depth. Linear-in-dB gain, which means the decibel gain is a linear function of the

There are multiple approaches for a TGC control circuit that are based on the type of DAC. Figure 8-4 shows a

high-level block diagram for the topology using a current-output multiplying DAC (MDAC) to generate the drive

low-current noise density. The current density helps reduce the overall noise performance because of the DAC

output configuration. The DAC output can go up to ±10 V; therefore, the op amp must have bipolar operation.

The OPAx210 is used here because of the low-voltage noise density of 2.2 nV/√Hz, low-current noise density of

500 fA/√Hz, rail-to-rail output, and the ability to accept a wide supply range of ±2.25 V to ±18 V and provide rail-

to-rail output. The low offset voltage and offset drift of the OPAx210 facilitate excellent dc accuracy for the circuit.

The OPAx210 is used to filter and buffer the 10-V reference voltage generated by the REF5010. The REF5010

results in a current-to-voltage conversion. The output of the OPAx210 has a range of –10 V to 0 V, which is fed

to the THS4130 configured as a Sallen-Key filter. Finally, the 10-V range is attenuated down to a 1.5-V range,

with a common-mode voltage of 0.75 V using a resistive attenuator. See the 2.3-nV/√Hz, Differential, Time Gain

Control DAC Reference Design for Ultrasound for an in-depth analysis of Figure 8-4.

Used only if +/-15 V is not

available

OPA2210

OPA2210

REF5010

DAC8802

+15 V

-15 V

+13 V

-13 V

+5 V

THS4130

VCNTL-P

VCNTL-M

VCNTL (0 to 1.5V)

with

VCM (0.75V)

REF+

VCM

10V from

REF5010

Required

VCM as

per AFE

10 V

REF-

10 V

-10 V

FPGA or

Controller

Interface

SPI

Resistor

Divider

0.75 V

I-to-V Converters

Passive

Attenuator

2

kHz)

+5 V

+13 V -13 V

+13 V

-13 V

+13 V

+13 V -13 V

LM5160

+5 V to 12 V

TPS7A39

TPS7A47

Figure 8-4. Block Diagram for Time Gain Control System for Ultrasound

www.ti.com

OPA210, OPA2210

SBOS924F – SEPTEMBER 2018 – REVISED JANUARY 2021

Copyright © 2021 Texas Instruments Incorporated

Submit Document Feedback

21

Product Folder Links: OPA210 OPA2210