Texas Instruments ADS8910B, Руководство пользователя

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ADC
AINP
AINN
OPA625
+
±
OPA625
±
+
.
.
.
.
.
.
External
Differential
Source
Note: External source
must be balanced
V
S
(+) = -V
S
(-)
3
2
1
3
2
1
R
S
R
S
V
S
(-)
V
S
(+)
J4
J6
J7
1-k Ÿ
1-k
Ÿ
1-nF
10-nF
1-k Ÿ
2.2- Ÿ
2.2- Ÿ
10-nF
10-nF
OPA625_CM
1-k
Ÿ
1-nF
J3
Analog Interface
www.ti.com
6 SBAU268 – July 2016
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Copyright © 2016, Texas Instruments Incorporated
ADS8910BEVM-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
with 10-nF differential filtering that keeps noise in external cabling common. The
overall signal path bandwidth is limited to 160-kHz by the anti-aliasing filter formed from 1-k Ω resistor and
1-nF capacitor at the amplifier feedback. Finally, the two OPA625 operational amplifiers drive the
ADS8910B differential inputs with 2.2-
Ω impedance up to 7-MHz that properly drives the low dynamic
impedance of the ADC inputs at 1-MSPS.
Figure 1. OPA625 Differential Input Driving Path
2.2.2 Input Common-Mode Jumper Configuration
The ADS8910BEVM board accommodates three external source common-mode options: 0 V, 2.5 V, and
floating with jumpers J1 and J2; see
Figure 2 .
J2 selects the OPA625 common-mode as 2.5 V (J2:OPEN) or 1.25 V (J2:CLOSED). J1 increases the
OPA625 common-mode by almost 100 mV to avoid amplifier output saturation with full-scale external
source signal amplitude. R1 is installed as 280 k
Ω , allowing full-scale external source signals for external
source impedance (R
S
) between 0
Ω
and 32
Ω
, with 0-V common mode. R1 must be changed to
compensate for larger external source impedance (R
S
) values or for 2.5-V external source common-mode,
as explained in Section 2.2.3 .