THS4551 Datasheet, PDF (39/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

www.ti.com
THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
Device Functional Modes (continued)
9.4.2.2 DC-Coupled, Differential-Input to Differential-Output Design Issues
Operating the THS4551 with a dc-coupled differential input source is very simple and only requires that the input
pins stay in range for the dc common-mode operating voltage. The example in Figure 76 takes the output of a
dual precision op amp (such as the OPA2192) where a high differential input signal is attenuated by the
THS4551 down into the range of an 18-bit SAR ADC such as the 2-MSPS ADS9110. The input stage provides a
differential gain of 21 V/V with a common-mode gain of 1 V/V. This example amplifies a small differential signal
on top of a very-wide range common-mode voltage. The input common-mode voltage appears at the outputs of
the OPA2192. The input common-mode voltage is level shifted by the FDA common-mode control to be at the
required output common-mode voltage to drive the ADS9110 SAR ADC (with a 4.096-V reference, as shown in
Figure 76); the FDA output common-mode voltage must be at the 2.048 V shown in Figure 76. This design offers
a very high CMRR using the common-mode control loop of the FDA to reset the output common-mode voltage
from that delivered to the inputs of the OPA2192. The actual CMRR from the OPA2192 inputs to the FDA
outputs is dominated by the resistor mismatches in the FDA. The feedback and differential input capacitors are
included to shape the noise gain as described in the Designing Attenuators section.
10 k
+
4.096 V
±
2.048 V
VOCM
10 k
1 µF
VIN1
VS+
5V ⁺
±
VS-
0V ⁺
±
VCC
15 V ⁺
±
VEE
-15 V ⁺
±
VIN2
Gain of 0.2 V/V,
DC-Coupled,
Differential Input to
2k
Differential Output
VCC
±
+
RG1
4.99 k
VEE
200
OPA2192
2k
49 pF
VOCM
VCC
±
+
VEE
RG2
4.99 k
THS4551 Wideband,
Fully Differential Amplifier
22 pF
RF1
1k
VS+
±
+
FDA
±
+
PD
VS- ^VS+
RF2
1k
ADS9110
10
Inputs
12 nF
10
22 pF
Copyright © 2016, Texas Instruments Incorporated
Figure 76. Example DC-Coupled Differential I/O Design from a Precision Dual Op Amp to an 18-Bit SAR
9.4.3 Input Overdrive Performance
Figure 12 illustrates a 2X overdrive triangle waveform for the THS4551. The input resistor is driven with a ±9-V
swing for the gain of 2-V/V configuration in the test circuit of Figure 61 using a single 5-V supply. When the
output maximum swing is reached at approximately the supply values, the increasing input voltage turns on the
internal protection diodes across the two input pins. The internal protection diodes are two diodes in series in
both polarities. This feature clamps the maximum differential voltage across the inputs to approximately 1.5 V
when the output is limited at the supplies but the input exceeds the available range. The input resistors on both
sides limit the current flow in the internal diodes under these conditions.
Copyright © 2016, Texas Instruments Incorporated
Product Folder Links: THS4551
Submit Documentation Feedback
39