Texas Instruments ADS9224REVM-PDK, Руководство пользователя, Страница: 6 / 34

AINP
AINN
3300 pF
4.32
-
+
+
-
1 k
THS4551
V
OCM
= REF/2
2.048V
-1.828 V
+1.828 V
1 k
0 V
0.22 V
3.876 V
2.048 V
-3.876
V
+3.876 V
0 V
5V
A0(-)
FSR_ADC
±V
REF
=
±4.096V
1 k
1 k
4.32
10
10
100 pF
-1.828 V
+1.828 V
0 V
A0(+)
0.22 V
3.876 V
2.048 V
100 pF
ADS9224x
AINM_A (Pin 2)
AINP_A (Pin 1)
AINP_B (Pin 8)
AINM_B (Pin 7)
SMA J2 (IN-)
Header JP1.1
SMA J3 (IN+)
Header JP2.1
THS4551: Fully-Differential Amplifier
SMA J4 (IN+)
Header JP3.1
SMA J5 (IN-)
Header JP4.1
AIN-_A
AIN+_A
AIN+_B
AIN-_B
-
+
+
-
5V
100 pF
100 pF
1 NŸ
1 NŸ
1 NŸ
1 NŸ
THS4551: Fully-Differential Amplifier
-
+
+
-
5V
100 pF
100 pF
1 NŸ
1
NŸ
1 NŸ
1
NŸ
4.32 Ÿ
4.32 Ÿ
4.32 Ÿ
4.32
Ÿ
3300pF
3300pF
Analog Interface
www.ti.com
6 SBAU315A – July 2018 – Revised June 2019
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ADS9224REVM-PDK
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 k
Ω
. The overall signal-path bandwidth is limited to 1.5-MHz by the 1-k
Ω
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.
Figure 1. THS4551 Differential Input Driving Path
The ADS92x4REVM incorporates two THS4551 FDAs to drive the ADC inputs. The FDAs shift the signal
to the appropriate common-mode voltage level.
Figure 2 shows the fully-differential amplifier circuit. A
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
OCM
input pin. The
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
OCM
input pin. The THS4551 shifts the signal
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.
Figure 2. THS4551 Fully-Differential Amplifier Driver