ADA4941-1_15 Datasheet, PDF (17/25 Pages) Analog Devices – Single-Supply, Differential 18-Bit ADC Driver

ADA4941-1
In this case, the linear output voltage is limited by A1. On the
low end, the output of A1 starts to saturate and show degraded
linearity when VOP approaches 200 mV. On the high end, the
input of A1 becomes saturated and exhibits degraded linearity
when VIN moves beyond 4 V (within 1 V of VCC). This limits
the linear differential output voltage in the circuit shown in
Figure 49 to about 7.6 V p-p.
665Ω
1.02kΩ
3
1kΩ
+5V
1 FB
V_S+
1kΩ
8 IN
A1
V_S–
REF
A2
6
402Ω
2
500Ω
OUT+ 4
+
VOP
–
OUT– 5
+
VON
–
VIN
+2.5V
Figure 50. 5 V Supply, G = 5, Single-Ended-to-Differential Amplifier
Figure 50 shows a single 5 V supply connection for G = 5. The
RF and RG network sets the gain of A1 to 2.5, and the 2.5 V at
the REF input provides a centered 2.5 V output common-mode
voltage. The transfer function is then
VOP − VON = 5(VIN) − 5 V
(8)
The output range limits of A1 and A2 limit the differential
output voltage of the circuit shown in Figure 50 to approximately
8.4 V p-p.
DC ERROR CALCULATIONS
R_F
R_G
R_S–IN
I_BN–A1
1 FB
I_BN–A2
1kΩ
8 IN ^A1
REF
V_OS–A1
I_BP–A1
2
R_S–REF
500Ω
I_BP–A2
1kΩ
A2
V_OS–A2
OUT+ 4
+
VOP
–
OUT– 5
+
VON
–
Figure 51. DC Error Sources
Figure 51 shows the major contributions to the dc output
voltage error. For each output, the total error voltage can be
calculated using familiar op amp concepts. Equation 9 expresses
the dc voltage error present at the VOP output.
VOP_error =
[ ] _{}1+
R_F
R_G

 ^{VOS _A1 − (I BP _A1)(RS _IN)}

+ (I _BP _A1)R_F
(9)
When using data from the Specifications tables, it is often more
expedient to use input offset current in place of the individual
input bias currents when calculating errors. Input offset current
is defined as the magnitude of the difference between the two
input bias currents. Using this definition, each input bias
current can be expressed in terms of the average of the two
input bias currents, IB, and the input offset current, IOS, as
IBP, N = IB ± IOS/2. DC errors are minimized when RS = RF || RG. In
this case, Equation 9 is reduced to
[ ] VOP_error
=
^{1 +}
R_F
R_G

V_OS _A1
+ (I_OS )R_F
(R_S = R_F || R_G )
Equation 10 expresses the dc voltage error present at the VON
output.
VON_error = −(VOP_error) + 2[VOS_A2 −
(IBP_A2)(RS_REF + 500)] + 1000(IBN_A2)
(10)
The internal 500 Ω resistor is provided on-chip to minimize dc
errors due to the input offset current in A2. The minimum
error is achieved when RS_REF = 0 Ω. In this case, Equation 10
is reduced to
VON_error =
−(VOP_error) + 2[VOS_A2] + (IOS)1000
(RS_REF = 0 Ω)
The differential output voltage error VO_error, dm, is the
difference between VOP_error and VON_error:
VO_error, dm = VOP_error − VON_error
(11)
The output offset voltage of each amplifier in the ADA4941-1
also includes the effects of finite common-mode rejection ratio
(CMRR), power supply rejection ratio (PSRR), and dc open-
loop gain (AVOL).
V_OS
= V_OS _nom +
ΔV_CM
CMRR
+
ΔV_S
PSRR
+
ΔVOUT
A_VOL
(12)
where:
VOS_nom is the nominal output offset voltage without including
the effects of CMRR, PSRR, and AVOL.
Δ indicates the change in conditions from nominal.
VCM is the input common-mode voltage (for A1, the voltage at
IN, and for A2, the voltage at REF).
VS is the power supply voltage.
VOUT is either op amp output.
Rev. C | Page 16 of 24