Ths4551 Differential Input Driving Path; Ths4551 Fully-Differential Amplifier Driver - Texas Instruments ADS9224REVM-PDK User Manual

Analog Interface
2.2.1 Input Signal Path
Figure 1 shows the signal path for the differential signal applied at the board inputs. The board input
impedance is 1 kΩ. The overall signal-path bandwidth is limited to 1.5-MHz by the 1-kΩ resistor and 100-
pF capacitor at the FDA feedback. The two THS4551 FDAs drive the ADS9224R differential inputs
through an RC charge-kickback filter. These drivers provide a low dynamic impedance source at the ADC
inputs at the full throughput of 3 MSPS.
AIN-_A
SMA J2 (IN-)
Header JP1.1
AIN+_A
SMA J3 (IN+)
Header JP2.1
AIN+_B
SMA J4 (IN+)
Header JP3.1
AIN-_B
SMA J5 (IN-)
Header JP4.1
The ADS92x4REVM incorporates two THS4551 FDAs to drive the ADC inputs. The FDAs shift the signal
to the appropriate common-mode voltage level.
differential input signal with a common-mode voltage of 0 V is applied to the inputs of the THS4551. The
FDA establishes a fixed common-mode voltage at the ADC inputs using the FDA V
ADS9224R incorporates a REF / 2 buffer output pin for setting the common-mode voltage. The
ADS9224R REF / 2 output is connected to each THS4551 V
to the required common-mode voltage of REF / 2. Because of the THS4551 output swing specification to
GND, either the input signals must be limited to a differential voltage of ±3.876 V amplitude in order to
avoid saturating the amplifier output, or the negative supply must be driven below GND (in other words,
–200 mV) to extend the output range.
+1.828 V
A0(-)
-1.828 V
+1.828 V
A0(+)
-1.828 V
6
ADS9224REVM-PDK
THS4551: Fully-Differential Amplifier
100 pF
1 NŸ
1 NŸ
5V
+
-
+
-
1 NŸ 1 NŸ
100 pF
THS4551: Fully-Differential Amplifier
100 pF
1 NŸ
1 NŸ
5V
+
-
+
-
1 NŸ 1 NŸ
100 pF
Figure 1. THS4551 Differential Input Driving Path
1 k
5V
0 V
+
V = REF/2
OCM
THS4551
2.048V
-
0 V
1 k
Figure 2. THS4551 Fully-Differential Amplifier Driver
Copyright © 2018–2019, Texas Instruments Incorporated
4.32 Ÿ
3300pF
4.32 Ÿ
4.32 Ÿ
3300pF
4.32 Ÿ
Figure 2 shows the fully-differential amplifier circuit. A
input pin. The THS4551 shifts the signal
OCM
100 pF
3.876 V
2.048 V
1 k
0.22 V
4.32
10
-
3300 pF
+
10
4.32
1 k
3.876 V
100 pF
2.048 V
0.22 V
ADS9224x
AINP_A (Pin 1)
AINM_A (Pin 2)
AINM_B (Pin 7)
AINP_B (Pin 8)
input pin. The
OCM
AINP
+3.876 V
0 V
-3.876
V
AINN
FSR_ADC
±V =
REF
±4.096V
SBAU315A – July 2018 – Revised June 2019
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