Datasheet

Bipolar Input to Single-Supply Scaling
R =
A
2 R g´ ´
B
1 g-
9.23077k =W
2 10k 0.315789474´ W ´
1 0.315789474-
R =
X
R R
B A
´
R +R
B A
4.81k =W
10k 9.23077kW ´ W
10k +9.23077kW W
R
10kW
B
+
R
X
4.81kW
R
A
9.2kW
CH1Input
(2.447817V,
0.0474093V)
V
IN1
(+5V, 5V)-
V
REF_ADC
(2.5V)
APPLICATIONS: HIGH GAIN/WIDE
k =k k
0.96=0.98 0.02
-
VO VO+ VO-
-
g=
k V´
VO REF_ADC
2 |V | k- ´´ V
REF_ADCIN1 VO
0.315789474=
0.96 2.5´
2 5 0.96 2.5- ´´
PGA112 , , PGA113
PGA116 , PGA117
SBOS424B – MARCH 2008 – REVISED SEPTEMBER 2008 ............................................................................................................................................ www.ti.com
Note that this process assumes a symmetrical V
IN1
and that symmetrical scaling is used for CH1 input
minimum and maximum values. The following steps
give the algorithm to compute resistor values for
references not listed in Table 12 .
d. R
X
can now be computed from the starting value
of R
B
and the computed value for R
A
.
Step 1: Choose the following:
a. V
REF_ADC
= 2.5V (ADC reference voltage)
b. | V
IN1
| = 5
(magnitude of V
IN
, assuming scaling is for ± V
IN1
)
c. Choose R
B
as a standard resistor value. The
input on-channel current multiplied by R
B
should
be less than the input offset voltage, such that R
B
is not a major source of inaccuracy.
R
B
= 10k Ω (select as a starting value for
resistors)
d. For the most negative V
IN1
, choose the
percentage (in decimal format) of V
REF_ADC
desired at the ADC input.
k
VO –
= 0.02
(CH1 input = k
VO –
× V
REF_ADC
when V
IN1
= – V
IN1
)
e. For the most positive V
IN1
, choose the percentage Figure 78. Bipolar to Single-Ended Input
Algorithm
(in decimal format) of V
REF_ADC
desired at the
ADC input. Since this scaling is based on
symmetry, k
VO+
must be the same percentage
away from V
REF_ADC
at the upper limit as at the BANDWIDTH CONSIDERATIONS
lower limit where k
VO –
is computed.
As a result of the combination of wide bandwidth and
k
VO+
= 1 – k
VO –
high gain capability of the PGA112/PGA113 and
PGA116/PGA117, there are several printed circuit
k
VO+
= 1 – 0.02 = 0.98
board (PCB) design and system recommendations to
consider for optimum application performance.
(CH1 input = k
VO+
× V
REF_ADC
when V
IN1
= +V
IN1
)
1. Power-supply bypass. Bypass each
Step 2: Compute the following:
power-supply pin separately. Use a ceramic
a. To simplify analysis, create one constant called
capacitor connected directly from the
k
VO
.
power-supply pin to the ground pin of the IC on
the same PCB plane. Vias can then be used to
connect to ground and voltage planes. This
configuration keeps parasitic inductive paths out
b. A constant, g, is created to simplify resistor value
of the local bypass for the PGA. Good analog
computations.
design practice dictates the use of a large value
tantalum bypass capacitor on the PCB for each
respective voltage.
c. R
A
is now selected from the starting value of R
B
and the g constant.
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Product Folder Link(s): PGA112 PGA113 PGA116 PGA117