Data acquisition
SETTING THE PGIA GAIN Selecting Gain and Feedback Resistors The gain and feedback resistors of the amplifiers should be precisely matched. The LT5400 quad resistor network offers 0.2ppm/°C matching drift and 0.01 per cent resistor matching over a wide temperature range as well as better common-mode rejection ratio (CMRR) than independently matched resistors. The gain resistors around the FDA also need to be matched precisely to achieve optimum CMRR performance. The LT5400 resistor network is used to set the gain of amplifiers. The gain computation is shown in equations 1 to 3.
By setting R1 = R4 and R2 = R3 when using the LT5400, the gain would be:
TABLE 1. PGIA DESIGN CONSTRAINTS AND KEY SPECIFICATIONS PGIA Specification
Design Requirement Output Common Mode Differential Output to ADC
Gains: Single-Ended or Differential
Supplies +VS/–VS Bandwidth Noise Offset Voltage Drift
The gain of amplifiers and the FDA (fixed gain of 2) make up the total gain of the PGIA as shown in Table 2. The LT5400 series has a variety of resistor options available as shown in Table 2. The ADG1209 multiplexer can be bypassed by using the amplifiers in a unity-gain configuration, so a total PGIA would be set to 2 in this case. To set the gain higher than 20, an external precision matched gain resistor (RGAIN) is added between the inverting inputs of the ADA4898-1 amplifiers, and the LT5400-4 are used as feedback resistors to achieve a target gain of 64 and 128 as shown in Figure 2.
To compute the RGAIN value, refer to equations 4 to 8.
SNR at 100kHz THD at 100kHz CMRR
Signal Chain Specification μModule Fully Differential
μModule Sample Rate 2.048V 8.192Vp-p 2, 10, 64, 128 +15V/–15V, –6V/–2V >50MHz <2nV/√Hz ≤2μV/°C >90dB for all gains Comments Fixed Fixed Programmable
A single supply of 5V can be used for FDA (trade-off: 3dB to 4dB SNR drop)
Required to drive SAR device at 15MSPS
Required to achieve SNR above 85dB
Overall lower drift reduces calibration burden in system
16-bit/18-bit 15MSPS
>85dB at G = 2, >73dB at G = 128
<–105dB at G = 2, at G = 128
<–70dB
Lower sample rate can be used if desired
Design target Design target
TABLE 2. LT5400 RESISTOR OPTIONS AND EQUIVALENT GAIN Device
R2 = R3 (kΩ) R1 = R4 (kΩ) RGAIN (Ω) ADA4898-1 LT5400-4 And the value of RGAIN for desired gain would be:
Selecting a Multiplexer A multiplexer is used to control various gains of this PGIA circuit by selecting the LT5400 quad resistor network. When selecting a multiplexer for this wide bandwidth discrete PGIA design, the multiplexer’s important parameters such as on
resistance (RON), on capacitance (CON), and off capacitance (COFF) should be considered. For this
Instrumentation Monthly November 2022 LT5400-6 LT5400-7 LT5400-8 LT5400-4 LT5400-4 1 1.25 1 1 1 5 5 9 1 1 N/A N/A N/A 130 63.4 6 5 10 31.77 64.09 12 10 20 63.54 128.18
Continued on page 42... 41
1 1 N/A
Gain (V/V) 2
4
Total PGIA Gain (V/V)
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82
